I do hope the new Mac Pro will be worth paying extra for! The current model always lagged behind with slow updates, and it seems that those who got a Mac Mini or an iMac actually got better machines for a fraction of the price.
Nowadays it seems that it's better to buy cheaper machines more frequently than expensive machines every once in a while. No matter how good your machine is, new technology will make it obsolete very soon (see Thunderbolt, USB 3.0, and of course new CPUs). Very often a cheap new machine will be as good as an old beast.
I think that to avoid obsolescence (as much as it can be), Apple should make the Mac Pro very upgradeable, maybe even more than the current one. Even a pro studio won't be buying new Mac Pros every 3 years, they might as well buy new iMacs every 3 years instead.