Aiden>1. For the same reason that putting Conroe and Woodcrest chips in the same Mac Pro line. If it's too expensive to have 2 separate logic boards for the Mac Pro, why would it make sense to have an entirely separate product for this?
2. Apple returned to profitability in the late 90s by *reducing* their product offerings, and they've held that pretty steady for nearly 10 years now. In fact, since they've moved to Intel, they've reduced their product line even further, axing the eMac from the consumer market and reducing the number of different portable offerings from 5 to 3 (4 if you count the black MacBook).
3. Every Apple product has a specific market in mind, and rarely do they overlap in what the product offers. This reduces confusion for customers, and probably more importantly keeps one product line from stealing from another, and reduces costs. If you have 2 high-end desktop systems, and their only differences are that one has slightly more expansion and slightly faster processors and is alot more expensive, why would people even consider purchasing the higher-end product unless they need to max out their PCI slots?
4. There isn't much of a market for PCI cards in Apple systems at this point. The reason Apple relies on 'dongles', as you put it is, again for simplicity's sake- FireWire and USB are a hell of a lot easier to install and configure than PCI cards, and at the time when Apple started going with less expansion (the introduction of the iMac), the market for expansion cards was getting worse, and is still pretty low. I definitely see a renaissance for those cards if Apple sales start to climb, and since OS X/Intel runs on 'standard' intel chipsets and uses EFI, companies like nVidia and ATi are going to be more likely to come out with drivers for OS X and box em with their standard video cards, instead of requiring video cards with a completely different bios to deal with open firmware. Which brings me to my last point...
5. Gamers are the x-factor in all this. It's pretty obvious that Apple came out with Boot Camp when they did to cause some buzz with OS X/Leopard and get gamers salivating about the prospect of running their gaming machine incased in brushed metal goodness. Based on the rumors that Mac Pros are coming out at WWDC and Leopard will ship at the same time as Vista, or before, it's pretty obvious Apple was giving PC gamers a come-hither look. I would certainly agree that the type of machine you are talking about would be of interest to mid-level gamers....folks that play lots of games, but don't throw around high-end Alienware money. So the question becomes if/when Apple targets the gaming market more seriously, what would be the best strategy to do so? Until Apple has more of a 'home-grown' gaming market, ie, they either start developing games themselves or bribe game developers to release games at the same time on OS X as they do Windows (and more importantly, release games that are *exclusive* to OS X, I would say that Apple isn't going to benefit much from diversifying their hardware offerings for gamers.
I'm sure you are right about Leotard. 
