Without choice, without variety, we would never be able to push the boundaries of what can be done. And that, in turn, would limit what can be done for and by end users. Having different toolkits doesn't break compatibility across different distributions so long as those distributions can support those toolkits. Heck, it's not even a problem with Motif anymore. There are even some that say we can't go on with competing distributions.
Competition is a good thing.
We get down to a few serious competitors supported by groups of loyal followers. In fact, they'll drive each other to work for improvements. In his March 2000 Linux Magazine article on this very topic, Dave Whitinger pointed out that the community tends to fracture into hundreds of similar projects. These environments are targeted at different users running in different computing environments. Blackbox on my big bad graphics system is boring, while Enlightenment on the Pilot would be overkill.