You guys love to pick on Burnett, but we arent talking the same issues here.
First of all, Smoltz is a different beast altogether. The guy is one of the toughest mo fo's you are gonna find. But his once 98mph fastball was down into the 92 range prior to his arm injury. Now, after reconstructive shoulder surgery, he will not even be ready if everything goes well until mid May at the earliest. How effective will he be?
Brad Penny was a guy who had flashes of brilliance in Florida but had intermittent injuries. Got to LA where he was less brilliant but much for durable for 3 yrs (29, 33, 34 starts respectively). Then, last yr, he has a shoulder injury that saps him of all his power and allows him to get slapped around the park. Who knows if he is even healthy?
Then we get to Burnett. The only one of the trio who is not currently injured. He gained a lot of notoriety for being injury prone in Florida, but since his tommy john surgery, he has actually been pretty durable.
2005- 32 starts- 209IP
2006- 21 starts- 135IP
2007- 25 starts- 169IP
2008- 34 starts- 221IP
Compare this to your current ace (Beckett) and AJ has averaged 6 innings less per season since 2005 (183IP per season). So, while he isnt the most durable pitcher out there, he has shown a significant improvement in his durability since recovering from Tommy John surgery.
Another thing I will say about Burnett is that, when he pitches, he dominates. There are some pitchers who will take the mound and suck if they arent 100%. Burnett doesnt do that. Granted, it doesnt sound like he pitches at anything less than 90%, but you know when he takes the mound, he's gonna be solid. I'll take that. Nothing worse than a Clemens outing in the playoffs where he's 50% and is only gonna hurt the team.