Tebow has one system he can operate in and it has to be specifically tailored for his very limited skill-set. He can't read defenses, his arm us way below average, his footwork is abysmal, his throwing motion is erratic and incredibly long. Nothing he does is repeatable so he has no earthly idea where the ball is going. Every single thing he does as a quarterback is counter to long-term success.
With the death of the read-option as a primary offensive staple in the NFL (frankly, it never had much life to begin with), there isn't a single team in the NFL that runs an offense that Tebow fits directly into. I guess they could further dumb down the neanderthal offense they run in Carolina and play truly ground and pound football if they wanted to, but while Rivera's offense isn't complicated, it DOES rely on the threat of Newton's strong arm to keep safeties back. Tebow doesn't have that so safeties would stack the box and Tebow would probably fail even in Carolina.
No, Tebow's a miserable backup option because the moment your starter went down, you'd have to scrap your offense and install a new one for him to be effective. And ultimately, the ceiling of 'effective' wouldn't be terribly high. Peak Tebow efficiency would be a bottom 1/3 offense in terms of productivity. I watched Tebow in person in Arrowhead and the only reason he wasn't picked off a half dozen times is because he was literally too inaccurate to even be in the neighborhood of defensive backs. He was the single worse pure passer I have ever seen and I've seen a bunch of them by now. Nobody is even in the ballpark.
That kid can't play quarterback in the NFL. He can't even come close. You need a backup that can run your offense for 2-3 weeks. Or if you're willing to invest in a long-term option that could REPLACE your present starter in the event that he goes down, you do that and tailor the offense for the new guy. But you don't bring in someone as a pure backup option with no path to starting who needs his own offense to be able to succeed.
Tebow sucked. He's not in the NFL anymore because he sucked. He won a few games because the Broncos defense was just turning into the fire breathing monster it became and because the AFC West was a cesspool that year. If he took the reigns of an NFL team, he'd be the worst quarterback, starting or backing up, in the entire NFL and by a pretty significant amount.