As a Gym Leader, I would not want the Clause in effect, since that's how I personally stop stat ups a lot of the time. But yeah, I don't think it's that great of an idea in other gyms or battles either.
I would have to say no to having this clause. There are ways around phasing.
Maybe something that makes it so a basic is optional if it's chosen in DPPt? (and if that happens, then if the mon is refused it can't be brought into the battle later; you've already said you basically don't have it in the battle.) The only real problem I see is the DPPt issues with basics, and that would deal with that without utterly ruining the moves for strategic purposes.
Accuracy and Evasion are less easily countered, because they have no drawback for the user. If you send out a fast mon, there is next to NOTHING the opponent can do to not guarantee an acc drop or eva boost, except for maybe Protect, but all that does is delay at best. Phasing has a low priority, so all it takes is for you to use some skill and predict they're going to phase, and Taunt them. Whereas, if you're hit by an Acc dropping move, or they raise their Eva, it's extremely hard to reverse it.
Anyway, I said that's why it's MOSTLY not based on bad luck. Even so, there's nothing stopping you switching back out again. Failing Taunt, possibly Torment to stop Dragon Tail spam? Teach them to your mons and Imprison? So many counters.
~The Artist Formerly Known As PichuBoy~
In my head I just saw you do that thing where you sashay with your fingers in the shape of a 'Z'. It was the best moment of my day.
You're talking to me about talking down to people having an opinion? The person that everyone hated for about 2 weeks FOR being vocal about their opinion? Do your research, please. I posted my immature side, then gave my serious points. What's the problem with that?
Uh, but seriously, let's just everybody chill out. This isn't a thread about that, it's about this clause, so erm. Yeah.
Anyways, I'm all for Siless' idea.
You had a new dream, it was more like a nightmare.
You were just a little kid, and they cut your hair,
then they stuck you in machines, you came so close to dying.
They should have listened, they thought that you were lying.
-Two by The Antlers
You can always Taunt, Sub, use one of the never miss moves. There's plenty of Pokemon with abilities related to accuracy.
I'm done arguing though, since no one else wants the clause.
I've always been favour of some way of protecting people from having basics dragged out in D/P rulesets, but I still wouldn't support a clause that totally bans the move, sorry.
The fact that they are a reduced priority makes them counterable. Roar and Whirlwind can be taunted. Dragon Tail and Vital Throw can be subbed against.