cessnapete, as I have said innumerable times already today: Just because you can't see it doesn't mean it's not there. If you spent some time flying through the stuff (i.e: across Britain or transAtlantic) then your aircraft would notice the effects.
I doubt that there's enough ash aronud to actually bring an aircraft down due to engine failure or anything like that, but it's better to be safe than sorry. I know that all the people complaining that this is an overreaction would be the first to complain about 'underaction' should anything go wrong due to aircraft ingesting ash.