Need a new computer? Perhaps every 3 years. (If you can get 7 years out of a Mac Pro, you probably didn't need it in the first place 😛)
Want a new computer? Perhaps every 6 months! 😀
Absolutely true. But if your needs are centered around good performance, though not top speeds, and expandability (i.e., you don't have or want to set up a server but need a ton of storage/scratch space or have, say, recording or digital gear that requires PCIx slots), a tower makes sense, so it comes down to whether you want to spend a little bit more for 2 towers during that seven year span, or just once on something that will meet your needs for 7 years. Either approach makes sense; the former requires you to spend a little more (though you might make that back on resale) and go through the buying/selling process, but you will end up with something that is a bit faster by the end of it, and plus you'll get that neat moment when you upgrade and everything is a bit faster than it was. Buying top of the line at first means you spend about the same and you don't have to worry about replacing for quite some time. Either meets your needs, so it's really your personal preference; I would say that in either example it would be silly to be buying the octo model (unless you get a good deal on it).
From my perspective, I tend to buy the best system I possibly can when I need to upgrade, so that I can do my job and not have to worry about upgrading until I reach the point of obsolescence. My needs are, again, still met by my G5, though I have replaced HDDs, upped RAM, upgraded the GPU, and added eSata to achieve what I need currently. Does this work for everyone? Of course not; I'd hate to have to, say, edit HD video with this box, as I doubt it would even be capable, much less adequate. So it's really again about wants and needs. If your requirements aren't for top of the line gear, a 2006 Mac Pro should work pretty solidly in 2011, and probably reach the NEED point around 2013-2014, again depending on your needs (and assuming there isn't another platform change that effects the software you need, like the full 64-bit change that's been mentioned several times here). I got lucky in that CS4 supports PPC still; I won't be so lucky next year.
Do I
want a Mac Pro? Oh, totally. But I can't justify the expense until I need it. If I made the kind of money where plunking down $2-4K wasn't a stretch for me, I already would have...