Trick plays rarely work, mostly because there doesn't seem to be a lot of trick to them. If they line up with a different QB before the play, the other team seems to get a good sense of what's about to happen, and the defense will execute an effective counter.
You can try it occasionally if you really want to, and every once in a while it will even work, but it doesn't justify putting a guy on the roster for that one purpose. I still don't think there's any reason for a team to sign Tebow unless they actually think he's worth his salary to be a backup who can step in and play decently if the starter gets hurt. IMO, you can probably get a better backup than Tebow for your money and sans all the drama.