Risk assessment with this is exceedingly difficult because of the lack of concrete information. However, using some arbitrary numbers and a bit of probability theory suggests that the closure isn't the overreaction some believe it to be.
If one assumes that the probability of a serious incident from an ash encounter under these conditions is one in a million; i.e. the probability of an individual flight making it to its destination is 999,999/1,000,000, then you can compute the likelihood of an incident over a given number of flights. You actually compute the odds of having no accident then subtract, but it yields the correct result.
Using this method, while admittedly arbitrary, does yield some interesting results. Over the span of only 25,000 flights, the chances of an incident are around 2.5%. For 50,000, it's 4.9%. Whatever number you use, it is clear that the risk is not negligible. Here, I've assumed that a given flight is 99.9999% likely to fly without having an incident. The numbers get a lot grimmer using lesser figures.
I think what this exercise does do is to demonstrate the difference in perspective between those in government, who have to look at the overall picture, and the individual pilots here who would launch under these conditions. Individually, the risk is seemingly small. However, cumulatively, a somewhat different picture emerges. This result is not entirely dissimilar to the "tragedy of the commons" problem in economics.