I don't have any problem with a sensei talking about how he feels we should behave...but if it were something I disagree with, I wouldn't feel compelled to follow his advice, and if I disagreed enough, I could skip his classes or leave the dojo. So I think it is risky for a sensei to go down this road; also, he should be sure his actions mirror his words.
I've seen senseis make a big difference in teenagers' behavior with a few well chosen words. But it can be abused and I would hope senseis are careful of what they say, and to whom they say it. Otherwise, they might do better to be quiet.
I've also seen senseis who just teach by example, you admire how they are and try to be that way, without them telling you. Personally, I like this way, but it may be because I prefer observing and feeling to being told what to do