I don't know about anyone else here, but I feel that the Enterprise crew deliberately going back in time to 1968 in "A:E", just for some historical research as opposed to some dire emergency, is a bit irresponsible as they risk damaging history with their very presence. And in "Yesteryear" they use the Guardian of Forever to do the exact same thing, for pretty much the same reasons (well, at least they do until Spock has to go back in time to save his own life). How was this kind of activity approved, knowing how dangerous the consequences could be?