The risk the airlines have to weigh up is the post encounter maintenance costs; it's not so much about avoiding multiple engine failures like the BA 747 over Indonesia – they were in high density cloud for quite some time.
One post ash encounter schedule I've seen includes: changing engine oil completely, a boroscope of the engines, pitot system tests and many more items within 50 hours. As the cloud density increases from low to medium, there is a lot more to be done – and sooner.
It grounds the aircraft for a while and the log-book entry will be used by a future buyer to push the price down.