Saying that using Yawn isn't violating the sleep clause because the opponent can switch out to prevent it is probably the most idiotic thing that I have heard all day. In fact, it is so dumb that I can't even think of a good analogy to describe it.
Yawn uses the threat of putting something to sleep to force switches. So if you use it, you are trying to either put someone to sleep, or force a switch. If your opponent doesn't want to switch, then you've violated sleep clause. If your opponent switches because if he didn't switch you would be violating sleep clause, then he's still undergoing a hazard that you inflicted upon him by making an attempt to violate sleep clause.
Just imo, I think Yawn could deserve an exception. As it doesn't directly put the opponent's Pokemon to sleep like Sleep Powder or something, there is a difference here. The choice of letting the Pokemon sleep now rests with the opponent rather than you forcing it onto them; they now have to either switch or take the sleep.
Actually, there isn't really a difference. If your opponent decides to take the sleep, then that's like you using sleep powder and it missing the first turn, and hitting the second turn. You make it sound like if the opponent stays in, he's inflicting the sleep on himself, which is bullshit.
Okay, I thought of an analogy. What if I told you that you have a choice. You can either not eat anything for a week, or you can go get something to eat, but I get to cut off your arm if you do.
By your reasoning, the choice of getting your arm chopped off lies with you. You probably don't want to lose your limb, but you probably also don't want to starve, either. Now how is this fair? I mean, it's probably against the law or something.
The other situation isn't fair either. And if you say, "well, really switching isn't so bad, i mean, likening it to getting your arm chopped off isn't fair..." well, oftentimes, people wouldn't want to try to force the switch if it weren't going to hurt their opponent somehow, like negating stat-ups, or giving themselves a free turn to set up.
Now, don't get me wrong, I'm not condemning phaze moves, I'm just condemning using phazing moves in a way that violates one of the standard clauses under which everyone plays.