Is he better than a lot of backups? You could make that case, definitely, but I think you're misunderstanding what teams look for in a backup. Nobody's going to waste time installing an entirely different offense for their backup QB, so Tebow's only viable if he's backing up on a team that runs an offense that he's capable of excelling in. Which pretty much nobody does, since he's a very unique player with very unique strengths and severe limitations.
This is where his terrible form and resulting inconsistency really kills him. If he had dedicated himself to fixing his fundamentals to the point that he could reliably make all of the basic throws, I think that at the minimum he would have a shot at backing up RG3 once Cousins gets traded away. Tebow would be a great insurance policy for the Redskins, and he wouldn't even be asked to make a lot of complicated reads (much like Griffin isn't).
But his throwing mechanics place such major limitations on him that he's only going to succeed if you install a custom-built offense specifically for him. The 2011 Broncos were able to do it on the fly, so it's not like it can't be done, but most teams aren't going to want to do that. Especially if what they're looking for in a QB is a guy who can win a couple of games if their starter goes down with a non-season-ending injury.