Dan doesn't like it. :)
Well, there are actually a lot of disadvantages, but that's the only important one, so it's probably not worth much thought over alternate threading schemes for Parrot at least--it's going with an OS-level preemptive threading model.
If you can ensure me that the hooks will be available in critical routines (blocking operations) to allow proper implementation of cooperative threads in a perl module, then that's all the support from the parrot VM I need :-)
I just hope you won't make my non-preemptive-threaded applications slower with your built-in support for preemptive threads :-)
-- Matthijs van Duin -- May the Forth be with you!