In front of one of the league's best defenses, Tebow managed to go 7-4 as the Broncos starter in 2011. This after the team starter the season 1-4 with Kyle Orton as their QB.
Tim Tebow was by no means a Pro Bowler, but he did manage to score 18 TDs (12 pass, 6 rush) in those 11 games. He only threw 6 interceptions, which is good, but he also completed less than 50% of his passes, which is bad. He ran for 660 yards, collecting 5.4 yards per carry, which is good. However, he also fumbled the ball 13 times (losing it 6 of those times), which is bad.
The fact of the matter is this: Tebow is a throwback QB, and not a great option as your starter, but his main issue as a stand-in QB is one that is fixable (fumbles) and Mike McCoy managed to put up a winning record and win a playoff game as the Offensive Coordinator of Tebow's offense.
Should the Chargers sign Tim Tebow? If they're willing to cut every other QB not named Philip Rivers, then yes. It would likely save them some money too, because he would definitely sign for the veteran minimum (plus some bonuses that wouldn't kick into effect unless Philip Rivers got injured) and would be just as useful as Kellen Clemens.
However, Tebow brings an awful lot of media baggage with him, and this is a tricky time during the season to be screwing with the chemistry of the team, the QBs, and the role the media plays in all of it. So, while it may make sense on paper, it probably doesn't make enough sense for it to be worth the headache.