That's a big problem with Apple nowadays. They offer two desktops that don't have a built-in display, and there's a huge gap between the humble Mac mini and the Mac Pro. And I fear it's getting worse now that they focus so much on the mobile market.
This has been a problem with Apple since at least 2005 and really since at least 2002, when I started thinking about getting a Mac: they've never sold a decently powered, expandable box.
And it probably will continue to be a problem moving into the future.
Apple tries to push external solutions to expandability, i.e. USB and wireless (and failed with Firewire). This works for some stuff (hard drives), but it's still a big waste if you want to get a faster (GPU, CPU) iMac, but would be perfectly fine with your existing display. At least used Macs sell at a reasonable price…
And Mac Pros aren't consumer machines. They might look like medium to high grade desktop PCs, but Apple tends to see them more as the successor to SGIs and Sun workstations.
The problem is: Is there a real demand for a mid-range desktop? Even the normal market is migrating towards laptops. And they have the advantage that the power users see them as more disposable than laptops anyway, despite the price premium, and that even low-performance models are a good value for those that don't need the speed.
The only possible exception where I'd see some money coming it would be gamers. Steam is porting more and more games, so there'd actually be some usage scenarios, and the Mac hardware landscape isn't unified enough (GPU and CPU-wise) so that developers could gain much by optimizing one path only (as done with gaming consoles). But alas, the usual home-built gaming PC is almost on par with a Mac Pro anyway (or even superior), so what would Apple sell for that? And they wouldn't earn a lot from upgrades, as those are sold by third parties.
So as much as I'd like to see a bigger product palette from Apple, I certainly understand the reason why they're doing it like this. And it seems to work for them.
The bigger problem is that we don't have a good desktop hardware market anyway. There's no real competition. Linux doesn't sell hardware (in that range), there's no IBM/HP/Sun/SGI/Apollo Unix market, there's no Amiga, Atari or Be anymore. It's either parts, parts, parts or desktops/laptops not designed for one OS specifically – or Apple.
We need a couple of new Performas...