Anecdotal, but I've never spoken to anyone who told me about a problem definitely being solved by an IG. What I have heard a lot of was that when equipment is plugged into seperate IG circuits and connected with non-isolated data-com lines, there can be circulating ground current. Or in the event of a minor fault, the parallel path presented by the data-com lines can actually cause fires.
I can see this potentially happening with reqular equipment grounds, too, but I don't wonder if the main problem is IGs are relatively higher impedance.
My opinion today is that they're smoke-'n'-mirrors.
-John
