I find it useful sometimes if one of them has acted in a wacky way. Once my old therapist had been acting very strange- he told me later he had been angry with me. I very much regard it as his problem that he had been angry, and it didn't help all that much to be told that, but the useful aspect was that at least I had an explanation for it.

My current therapist also did something really stupid once, and when I brought it up to him a few months later (we had talked about it at the time as well), he explained what was going on in his brain (he was having a problem that had nothing to do with me but it had been distracting him). After I heard him own that I could just set it aside as a misstep based on his own stuff, so it no longer bothered me.

I guess I view it as useful as an explanation of weird behavior, but I'd prefer they keep a more even keel so it isn't necessary.