Various news reports indicated a NATO aircraft (reportedly a Belgian F-16) was damaged when flying through the volcanic ash associated with the recent eruption in Iceland.

While it may seem hard to believe, the airspace restrictions following the eruptions were not a publicity stunt.  For example, Air Force regulations specifically prohibit flying in the vicinity of eruptions.  From AFI 11-202v3: 

5.25. Volcanic Activity. Air Force aircraft will not be flown in an area of known or reported volcanic activity unless engaging in operations (such as rescue) specifically relating to the incident. Any encounters with pyroclastic clouds (volcanic ash) should be reported as soon as possible to the appropriate controlling agency (ARTCC, military radar unit, AWACS, etc.). In all cases, follow with a report to a PMSV, or other applicable weather agency, to ensure rapid dissemination to other using agencies. See PIREP procedures in the FIH.

Unlike flying through rain clouds or even most dust storms, the ash associated with a volcanic eruption contains particles that can be extremely hazardous to aircraft.  The result can be as benign as stripped paint and damaged windshields, or it can be as bad as the loss of all engines (or engine, as in the single-engine F-16 reportedly damaged).  In addition, the static electricity and subsequent lightning generated by the clouds (already seen in some press photographs) is another threat to safe aircraft operation.

There are unique hazards in aviation; while it is possible to be overcautious, one should be averse to simply dismiss the advice of those with vast experience in the field.