If Dobbs gets injured, you simply put in one of the redshirts. If the game is in hand and you want to avoid injury, you put in the walk on.
So, a (hopefully fictional) scenario to consider. We redshirt all three freshman. First five games of the year, Dobbs is starter and sophomore walk-on Devin Smith is backup. Smith gets considerable play time in the Bowling Green, Western Carolina and Florida games, as all three are blowouts by the 3rd quarter. By the Arkansas game, Smith has played almost five quarters of football, more than 50 offensive plays. In the Oct 3rd Arkansas game, Dobbs sprains his elbow and will be out for up to a month. We face Georgia on Oct 10th, six days from now, and Alabama is a couple of weeks later. Not to exaggerate, but the season turns on these next two games, between a mediocre year and a run at the SEC east title.
Who do you start? The sophomore who has 5 quarters of live game experience under his belt, but is (to be blunt) not talented enough for a run through the SEC? Or a true freshman who has never played a single snap of college football?
Both awful choices, right?
Game experience is worth gold, and you can expect to see some of that gold in at least a couple of the early games of the season. Give that to the one you REALLY want to be your backup in case of injury.
That's how I see it, anyway.