I think that often the difficult thing with telling someone you aren't interested, is that the reason you aren't interested will hurt them. Ie, telling the person bluntly "I can see that a life with you would drive me insane," is probably going to emotionally devastate them.
At the same time, I think honesty in these situations is worthwhile... So I don't really know. Tbh, thinking about it, I do actually think that if you are in a relationship, an explanation is generally owed - Though I don't really like the word "owed" I think I'd prefer, "should be given." I just think it's mean to not give one. All depends on the context of course, and I think the longer things have been going on, the more likely it is that an explanation is a good idea.