Well - the actual component swappability of a Pro in terms of practical usefulness is open to debate, once you get around the fact that it has slots, especially for a consumer-orientated user.
There are however many good reasons why you wouldn't want to build a PC as the alternative - especially if you lack the basic skills to build one properly, but believe otherwise. Many DIYers may disagree but I also suspect they've never been inside an upper-end PC before and looked at the tweaks involved in comparison to a DIY (unless that is you actually know what you're doing).
But there are many viable pre-built machines to choose from. Put it like this - a Dell XPS 730x for example will get you more power than the current single-socket Pro to start with for roughly the same money (including, if you want, a media suite to get you started, just like a Mac), and has a far better upgrade path from a consumer perspective in terms of what you can stick inside it without the hacks (many of which compromises the design ethos of the Pro, making them even more laughable) you see on this subforum. It is also ironically a more heavy-duty machine than the Pro, not to mention better paid-for supported than the supposedly 'Pro' machine.
What is even more ironic to me is that many of the uses which requires the most horsepower in a dual-boot situation for most people of a consumer orientation is actually in Windows, and many Switchers are blithely tying one hand behind their backs for the main use in terms of resource usage, purely for the supposed crushing superiority of OS X over what's shipping now on comparable systems - Vista64.
Actually, I have never been in such a upper echelon pre built system bc they tend to be very expensive (where I live) and don´t offer (afaik) anything in terms of OC which is one of the boons in building your own system
I was wondering what kind of tweaks and so forth are actually employed in such high end systems. I know what the pro looks like and I know what my usage patterns are.