Valerio Proietti has written a thoughtful post on why MooTools won’t use Sizzle (and tries to argue why other libraries shouldn’t).
I don’t expect every library will drop its own engine and plugin Sizzle right away. However, if done correctly, Sizzle can be an area of collaboration. If it becomes “John’s Library” that is one thing, but you will note that there have already been patches from the Prototype folks and maybe others. Sizzle could be a playground for innovation itself.