Addressing them out of order, the second point that I made is that cooperative multi-threading doesn't get you the ability to take advantage of multiple CPUs. (Or even multiple virtual CPUs, see Intel's hyperthreading.) That remains true.
The first point was that any blocking call anywhere blocks the whole program. Yes, there are strategies to avoid making blocking calls. However those strategies might not work in every case, and depend on everyone's cooperation. Even if a strategy can work, unless it works on every OS of interest, Parrot will have to implement the blocking forms. Furthermore in the real world, you have to accept that some extension authors will slip blocking calls in.
Therefore even though cooperative multi-tasking can work smoothly in theory, in practice it doesn't.
This claim isn't just cynicism on my part. Historically cooperative multi-tasking has been the first kind of multi-tasking that people reach for. After all it has lots of advantages. It is simple. You can trivially avoid lots of nasty synchronization bugs. In theory it can work wonders. But again and again experience showed that people don't all cooperate, and people have wound up biting the bullet and accepting pre-emptive multi-tasking.