There are many walls who do well against non choiced attacks, but can't handle such things. Magic Room will stop this. Magic Room will also allow you to send in your Trick pokemon with more effectiveness, them having some time before being forced into the trick, assuming choice.
Another option you missed is allowing choice pokemon to power up. Imagine a pokemon that only needs two moves for type coverage being choice scarfed. He gets the Magic Room off, he powers himself up, then when choice is working again, you ****. Or if you need three attacks, just set it up with another pokemon, switch him in, Sword Dance, Sword Dance, Sword Dance, and now choice scarfed at plus 6 attack, you just won the match. Gimcky setup, of course you wouldn't make it anywhere near that obvious, nor would you get to 6 lol, but a decent example. This will also allow Creselia to Trick away Flame Orb much easier.
This IS mainly against the opponent however. Hopefully enough pokemon get the move to make a bunch of viable set ups possible.