Could it be just a major part of the Wintel market?
After all, if Apple is seeing 40% growth in both desktops and laptops, their desktop offerings must not be as poor as is claimed by some? Especially since Apple's strongest retail growth is in machines that cost more then $2000 - a retail market they currently control two-thirds of?
A common argument is that Apple must offer a mid-tower because Windows users predominately have mid-tower models and therefore Apple must have one too in order to appeal to them to switch.
And yet what is the model Apple created specifically to attract Windows users? The Mac Mini. And Apple has done very little to it since they created it. About the only product Apple gets more stick about then the mid-tower is the Mini. A large number of Windows mid-tower owners evidently seem to want to use the Mini to become introduced to OS X and the entire Apple experience, even though it's small and it cannot be expanded. All they want is something newer then technology that wasn't cutting edge 24 months ago inside of it.
Even in the Wintel world, mid-towers are starting to fall by the wayside - especially amongst corporate customers. More and more companies are moving to small form factor PCs because they take up less space, draw less power and more then meet the needs of their users. Even retail Wintel boxes are getting progressively smaller over time. I travel a good bit for work and all my suppliers (and these are big companies with thousands of desktops) are all either using SFF machines or are replacing their towers with SFF machines to a very heavy extent.
The biggest market going forward for mid-towers will be the enthusiast market who either build them themselves or want them for very narrowly-focused purposes - like gaming or media center rigs. Those who build it themselves do so for both the challenge and the labor savings, so an Apple mid-tower would not appeal to them since Apple builds it for them and therefore no labor savings can be obtained. And those who want a gaming or media-center rig will be disappointed both by the lack of high-performance gaming cards available and Apple's desire to serve media content from the web - not a big box of disks in the closet.
If Apple is seeing 40% growth in its desktop products, that doesn't mean the offerings aren't bad. There could be bigger growth figures if Apple directly appealed to its switchers with a MID.
The small computers businesses are migrating to are often thin-clients. Yes, I know this isn't the 90s, but thin clients are still becoming more popular. This is due to more applications being web-based, and businesses centralising processing. So you could have a cheap client with a powerful server, and use terminal services to use the server's power remotely.
Power consumption isn't a major concern. The biggest savings would be in changing their air-conditioned servers. Those things take massive amounts of power, are often on 24/7, and require large amounts of energy just to cool them down.
The Mac Mini is a poor offering and doesn't generate many sales. That's the reason Apple haven't updated it. There's no reason to - not because it isn't selling well, but because it's a product with no clear target market, and refreshing it wouldn't help change that. Families don't buy small macs to experiment with them. When they buy a new computer, they buy something they're going to keep. It's also psychological - the mini is so small, people don't believe it'll give them the performance they need. They don't know it's way powerful enough to browse the web and make home movies. They've been told for years by Dell that the towers are necessary for performance. Bigger = better. It's not correct, but that's what lots of people believe.
Apple work by providing limited functionality, but doing whatever they do implement really well. Microsoft do the opposite - they try and do everything, and end up not being very good at doing anything. People are used to Microsoft's way of doing things. It's a huge mentality shift. It's the reason people don't like Apple products that can't open up, even though they've never tried to do it with their PC. You can't sell an MS-Mentality person the idea that they don't have the option to do something they won't do anyway. They're used to lots of options.
The mentality shift is one of the reasons Apple products have a 'halo effect'. As people use the iPod and iPhone, they like the product's focus on quality, and realise the lack of options isn't a problem. A MID would help bridge the gap even further.
