First of all, to Fooch/mods: I hope this is a narrow enough topic for a FanPost. It's my first time writing one. :)
With regard to the 49ers' current quarterbacks, it appears that the top two slots are set in stone (barring injury). Alex Smith WILL be the starting quarterback on opening day, with Colin Kaepernick as his backup. Currently, the 49ers are carrying two other quarterbacks in camp, both undrafted rookies: Jeremiah Masoli and McLeod Bethel-Thompson (henceforth referred to as McBLT). Neither of these two is considered a lock (or even likely) to make the final roster.
Now, the heart of the issue is whether or not the 49ers should bring in a veteran quarterback to fill the #3 position behind Colin Kaepernick. Many Niners Nation commenters (and radio personalities) have espoused the position that Josh Johnson should be brought in from Tampa Bay. It has been suggested that his familiarity with Harbaugh and his offensive system would be a boon for him were the 49ers to acquire him.
My opinion is this: why NOT keep a green rookie as the third rostered quarterback? In the unlikely circumstance that both Alex Smith and Colin Kaepernick become injured and are unable to play, no veteran quarterback who was cut from a 53-man roster after the preseason is likely to be able to play at their level, or even close to their level. The players cut from the training camp rosters are usually poorly rated players - undrafted rookies and veterans alike who couldn't beat out the guy ahead of them for a spot. While there's often some gems to be had on the waiver wire, scrubs are the norm. In short, the season is (in all likelihood) lost, at that point.
In the 2010 NFC Championship game, Bears quarterback Jay Cutler and his backup were injured, so the third string quarterback had to play. Enter Caleb Hanie, a 2008 undrafted free agent from Colorado State. Several Packers players quipped that they'd never even heard of the guy before he came in and drove down the field, nearly leading the Bears to a game-winning touchdown before being intercepted inside the 20. Hanie had thrown 14 career passes prior to that game, but he still showed enough moxie to nearly bring his team back. This is what I'd like to see in San Francisco.
The 49ers should carry an undrafted rookie as the #3 quarterback. Doing so allows him a few meaningful practice repetitions (something he wouldn't get on the practice squad; see also: Nate Davis), and a chance to develop into something more than a green, undrafted, no-name scrub. If both starters are hurt, he will get starting time and can learn from that. He'll have gone through training camp with the team at least once, so he'll have a working grasp of the offense and be able to get better each year.
Basically, the way I see it is: you've got "The Guy", "The Other Guy", and "Who Is That Guy?" Maybe, with hard work and a bit of luck, "Who Is That Guy?" can become "The Other Guy" or even "The Guy". Even though the 49ers appear to be set at quarterback for the foreseeable future (Alex for now, Colin for later), if you know you're probably not going to win games, why not develop a guy who could become a great starter or gain some trade value down the road?
What's your take?