I'd also guess it's okay, IF you use it merely to check out your own research. Not grabbing wholesale - and don't believe it without properly checking, in any case!
It's annoying though, when you see errors and kidnapped / mixed up families on trees - double the number of children for one couple, because as I knew, they'd mingled two different families in the same small area (I'd nearly done the same myself, ages before, then commonsense had kicked in and I'd checked it out more thoroughly)
Sending messages to point out firm errors or offer further information can be frustrating if you don't get any response - and months later you happen upon the same tree and they've done nothing. Makes you wonder if it's worthwhile trying to help out.
I feel a lot better, though, having sorted out things by myself, from records, I'll admit. Very little has been helped by viewing other peoples' trees - but loads of help given on "Rootschat" when I've been stumped. I always am reluctant to ask, wishing more often that I could more often give some help in the same way.