St. Clair isn't a great pass blocker, but at RT, he doesn't much have to be. Case in point: the Lions played Stockar McDougle at RT last year, who's a road grader in the running game, but is terrible in the passing game. The Lions gave up just 11 sacks all year. Unless you're blessed with a pair of awesome DEs (like say, Taylor and Ogunleye), there aren't many pass rushers at that LDE spot.
And I agree with you that the OL is what's important, more so than the glamour boy skill positions. I was hoping that Carey would be advanced enough at this point that he'd be at RT, and they'd be talking about St.Clair inside or as a 6th lineman. No such luck. However, if Carey goes inside, and Damion McIntosh can step in at RT for the second half of the season, that should make for a good line.