Tremonti is a LOT more than decent, and he's easily the most PROLIFIC guitarist of the decade, between the Creed reunion, Several Alter Bridge albums (some of them very good) and all of the Tremonti albums (4 of them - three of them excellent).
You also have to take into account that while I definitely believe Syn is the better guitar player, we have a deeper window into him here than the casual fan or guitarist gets. The Stage is my favorite album, but it didn't catch on like they hoped and also alienated a number of fans, as well, by being TOO COMPLEX for them. I say we're better off without them people, but as a numbers game, Mark's probably got the most visibility out there.
I sat down with Mark for an artist session about 5 years ago, and he's just as nice and humble and real as Syn is. Sure, I would have preferred Syn get the deserved recognition, but I can't fault the choice of Mark, either. He has definitely earned it.