Really? I didn't know cloaks grew on trees. I've never seen a party where more than one PC had a cloak of resistance...
Actually, thanks to MIC, there is no reason for every PC not to have the equivalent of a cloak of resistance. It is expected and necessary, in that the designers assumed you would have access to such basic gear when they designed their encounters.

They don't exactly have to grow on trees, but eventually, everybody in the party should have one. Unless of course, your fighter looks forward to failing every will save he comes across or something...