The Mac Pro is profitable. We know this, and we know it's actually one of the more profitable individual lines. Not iPad kind of profits, but not everything can be iPads. Admittedly, some of these profits are because Apple doesn't lower the prices of older hardware over time, and the Mac Pro hardware is indeed fairly old. But Apple does make a profit off the Mac Pro.
The Mac Pro does not sell as well as other Macs, of course, so while it makes good profit on each individual sale, it does not make much money on the Mac Pro market as a whole, compared to more popular products like the iMac or MacBook Pro. Still, profit is profit, and overall the Mac market is growing so presumably the Mac Pro market is growing as well, just perhaps not as quickly.
As much as some people like to argue about Apple's consumer-orientation or its various previous business and engineering decisions, there is simply no reason for Apple to drop the Mac Pro as a line. They might change the name or the form factor, but Apple will continue selling a product we'll recognize as an heir to the Power Mac for a long time to come. As I said, profit is a profit, and there's only one reason why you turn down profit, and that's if you can make more profit doing something else. This is the key question we have to ask. Can Apple make more profit by dropping the Mac Pro line?
The answer, of course, is no. Mac Pro customers will not buy iMacs if they lose the Mac Pro, nor will they buy iPads or MacBooks. The Mac Pro caters to a niche consumer, but no other device Apple sells fills that niche. Apple will be dropping the albeit small profits made by the Mac Pro market in exchange for nothing. Businesses do not simply reject money.
People talk about the Xserves, but the Xserve was a small niche product that had two competitors also made by Apple. It competed with the Mac Pro Server, and to a lesser extent it competed with the Mac Mini Server. In essence, the Xserve was cannibalizing or being cannibalized by other products sold by Apple in the niche. Apple rightly dropped the line to reduce development costs within the market, and focused their efforts on the high end Mac Pro and the low end Mac Mini which were both able to compete in two separate markets instead of just the one the Xserve could. For the Mini, a cheap headless Mac for switchers. For the Mac Pro, workstations for pros.
People talk about the MacBook, which has much the same story as the Xserve. It was a product caught competing against multiple other Apple products for the same set of consumers. Apple ended the MacBook line to focus their efforts into the MacBook Pro, MacBook Air, and I suppose even the iPad.
People talk about the Cube, and you can probably already tell where I'm going with this. It was stuck between the iMac and the PowerMac G4. It was also rather expensive, which didn't help its case. The Cube was resurrected as the Mac Mini, eventually, but they are definitely not the same thing. Both are headless Macs, but what really matters is the price and power, and the Mac Mini found its niche in a way the Cube never could. The Cube has a better analogy with the xMac people ask for, the non-workstation tower we're likely to never see. At least, we won't see an xMac until Apple figures out a way to lure more customers away from the Windows world without cannibalizing sales of their iMac and Mac Minis (and even, Mac Pros).
The Mac Pro simply does not have the same problem the Xserve or other dropped lines have. It exists as the sole workstation in the Apple lineup. To kill it while it is profitable will simply mean a drop in profits for Apple, which is not something companies want to have happen. Furthermore, another company will sell workstations to those former customers, helping Apple's competitors, and those former customers will stop buying software from Apple, and their motivation to stay in the Apple Ecosystem of iPhones and iPads and an iMac at home for the kids and a PowerBook Air or Pro on the road.
Apple, especially recently with iCloud and stuff like Apple TV, wants to keep people in the Apple ecosystem. There may be a bigger halo effect from the iPod and iPhone than anything else, but you cannot deny that people who use Macs at work tend to be more than just one-time Apple customers. Everyone I know who has a Mac at work- any Mac, but Mac Pros especially- have multiple Apple products for use at home. What's more, they evangelize better and more consistently than the gadget freaks who may have an iPhone today but are ready to jump ship to the next greatest Android or Windows phone the next year. Most people are forced to use whatever they have at work, but they pick their other devices based on their own individual factors. Having a Mac Pro workstation in the office makes them a lot more likely to choose what they're used to for their computer at home, and whatever other devices work best in the ecosystem.
ROI, another term tossed around a lot, is good, but it's not the end-all, be-all of business. A company might sell oranges, and have the best ROI on those oranges as they can possibly have. They have magical orange groves that require no fertilization or irrigation, they just produces tons and tons of oranges for the minimal cost of picking and shipping them every year. But there's only so many oranges you can sell, only so much orange juice people are willing to drink and some people don't like oranges anyhow. A good company doesn't just sell oranges because it's the product with their highest ROI. They sell some pears too, because that's another market available to them they might be profitable in, even if the pears don't have as high an ROI and the market is smaller. It's also good insurance for when people's tastes change, or if there's a bad crop one year. A good company diversifies, cuts product lines that aren't profitable or are wasting money in opportunity costs, but keeps the profitable parts of their business alive, regardless of their individual shares of the overall profits.
Apple has always been, at least in recent memory, good at identifying where they need to focus their efforts, and good at what aspects of their business they need to change. The Mac Pro remains profitable in itself, is a vector for software and other hardware sales, and Apple makes no other product that can do what it does. Professionals have nothing at all to worry about until there are simply no more professionals for Apple to sell to, or Apple cannot sell a profitable professional machine. Talk of Apple evaluating the line shouldn't terrify anyone, a good company evaluates all of its product lines often. Somewhere at some point at some level, Apple discusses whether they should kill off or radically change the iPhone, the iPad, the MacBook Air, and all of their other products. This is a good thing, because when Apple discusses the future of a given line, that means they're trying to figure out how to make it better.
We'll see a refresh on the Mac Pro when the chips to put in a new one come out. Yes, it is Intel's fault, and hopefully in the future, Mac Pro suitable chips will be quicker to arrive. Apple could have stuck Thunderbolt! or USB 3.0 or whatever else in their old Mac Pros, but then people would be complaining about the CPUs remaining the same and really those peripherals don't exist yet in big enough numbers for anyone serious to care. Apple could not have put an i7 in there, though, and anyone who thinks otherwise does not understand what a workstation is. Yes, there are hobbyists and gamers who buy the Mac Pro as a regular desktop machine. Heck, I'm one of them. The Mac Pro needs dual-CPUs and ridiculous numbers of cores, though, and ECC RAM. The Mac Pro needs two optical drives and four HD bays, and tons of PCI slots. It needs a huge open grill on both ends so it runs quietly, and plenty of space inside so you can work with the internals easily for upgrades and hardware swaps. Open up a Mac Pro sometime if you have access to one, and really take a look. It's beautifully designed to be a pleasure to work with. Things slide in and out nicely, there's no cables to mess with unless you're working with really power-hungry GPUs, and everything is neatly packed in with plenty of room for air to breeze through and keep things cool and quiet. It's a machine built for professionals in a professional environment, don't think you can just slap a bunch of consumer/gamer parts into it and still call it a Mac Pro. It might meet some people's needs, but Apple would be abandoning a market that does need a workstation and those customers would have to go elsewhere.