After watching on the sideline for the last days, my 2 cents as someone directly involved:
1. Yes, the decision not to fly will be conservative. That is how we deal with unknowns in aviation. If you don't know what is going to happen, don't do it.
2. The reason why the decision is conservative: testing the maximum ash density (dependent on ash type) that may safely be flown through is expensive, very expensive. Luckily, until now volcano eruptions have been in such locations that we could fly around any suspected clouds. Furthermore, none have been in such density used airspace with such a densely populated area below it (not only the aircraft, crew, and passengers are at risk in a crash). Flying around was cheaper than spending R&D and investment in being able to fly through predicted ash conditions.
Rest assured that, now that avoiding ash is clearly no longer the cheapest option, researchers in all kinds of fields (primarily the engine manufacturers, but also meteorologists, risk management experts, aviation maintenance experts) will be going to spend larger amounts of money on this. This may result in better ash tracking techniques, the fitting of ash detection equipment on aircraft, operational limits and all kinds of inventions. Only situations like the present makes spending the money on this worthwhile.
3. Finally on the validity of the VAAC model. (meteorologists please correct me where appropriate) The model is simply the best model available. If you compare the various dispersion results from Germany, Noway and the VAAC, you will see they are very similar, which tells me that they are most likely quite good.
The model probably has been tested based on previous volcanic eruptions and other dispersal events like as Tjernobyl. This situation is quite hard to recreate in small scale or otherwise. The data from the NERC flight will be used to validate and correct the model were needed.
Now lets all hope first of all that that ugly piece of rock in Iceland decides to quit smoking soon!
00nix