John Siracusa, in a really smart piece wherein he vouches for a new Mac Pro:
On paper, the Mac Pro may no longer be a viable product, but it would be a mistake for Apple to abandon the concept that it embodies. Like the Power Mac before it, the Mac Pro was designed to be the most powerful personal computer Apple knows how to make. That goal should be maintained, even as the individual products that aim to achieve it evolve.
Agreed. While I have no use for a Mac Pro, I think Apple should keep it around for those who do.
See also: Marco Arment's thoughts on a "Mac Pro Mini".