I posted this link in the other thread .. but I believe it has some relevance to this discussion ...
http://www.flightglobal.com/articles/2010/04/16/340727/pictures-finnish-f-18-engine-check-reveals-effects-of-volcanic.html

Indeed, no one disputes that volcanic ash can damage engines. The problem is that no one seems to be measuring the amount of ash over our skies now. We do have a plane that could do it, I guess, the Facility for Airborne Atmospheric Measurement aircraft G-LUXE (a converted BAe 146), but it's undergoing heavy maintenance. Talk about bad timing!

TBH, I can see both sides of the argument. Fog, wind, snow etc. have caused more aviation casualties than ash, yet airspace is not closed in these situations. On the other hand, NATs, and others (especially in today's litigious climate) err on the side of caution.
To those who speak of commercial pressure, I don't think any of today's major airlines would risk huge lawsuits by flying in clearly dangerous conditions.
However, when all's said and done, I'm just sitting here spouting my take on all this. I have no responsibility for aircraft, aircrew or pax. There are far better qualified souls than I who are well-paid to take the decisions that need to be taken.
That's why there's a question mark on the thread title!