Competition for most spots is good I think. But when you're the triggerman for an offense I think the psyche is much more different then any position. Fine line between letting the starter know you are ready to take the reigns and all out competition for the starter role. Too much and you have a starter getting scared every time he makes a mistake he's going to get yanked so maybe he doesn't take that risk on the downfield pass or a matchup that has a slight edge. On the backside of that you've got a backup who is willing to take big risks to show he is a capable playmaker on the field and forces passes he shouldn't.
Looking around the NFL the biggest thing holding some teams back is stability at the QB position. The only team last year that made the playoffs with lingering questions at QB is Tennessee but they also decided to go ahead and commit the team going through the season under Kerry. If you've got questions in the offseason and you've got two QBs that have starter potential then let them battle it out. Then you commit to one of them as the starter and go from there.
The thing that bothers me about the QB competition crowd and Schaub vs Sage this entire time has been the glossing over the level of commitment the organization has made to Schaub. It would've been one thing had Schaub been brought in as a FA QB to battle with Sage for the starting position. That wasn't he case obviously because of the trade for Matt. You don't make a trade like that unless you are convinced that he's capable of not only being your long term starter but also he has the tools to get the team into the playoffs. How bad does it look to your team, fanbase and owner if you make a trade like that, under those premises, and then say okay the QB is officially open for competition!
At least that's the logic in my mind.