You're completely missing the point of the Mac Pro if you think the only innovation is the size. Sure, it may not be what existing Mac Pro owners want... but how do we know that being different automatically makes it worse? Here's a good write-up on just why the new Mac Pro *could* end up being a game-changer. It's not all about the CPU:
http://kickingbear.com/blog/archives/349
Of course, there's a chance that the new Mac Pro doesn't gain much traction. But you can't write it off when many similar gambles have paid off for Apple. Just as nobody needs this new Mac Pro, nobody needed a 10" iPod touch. The perception in 2010 was that if somebody already had an iPhone or an iPod touch, why would a bigger version of those be anything special? And if somebody already had a much more powerful laptop, why would they want an iPad either?
And that's where we are with the new Mac Pro. Nobody sees why it's necessary... but I have a feeling we'll find out why when people truly begin to make use of it.