True Guards like Robert Brewster and Travis Bright have been doomed to be inactive because of their inability to play Center. It's the same phenomenon that's kept Cory Procter around. Unfortunately, we cannot bring everyone to play on game day, and you just don't need very many o-linemen for special teams purposes. This has made versatility very important among backup O-linemen. It seems that, as of late, it's been easier to find a C/G than it has to find a G/T.
Presumably, Brewster and Bright are pure guards. They don't have the ideal height to play tackle and it's important to have players on the edges who can keep the QB clean.
I think that the Cowboys' recent reliance on a C/G backup could change this year for several reasons:
1) we might draft Maurkice Pouncey, and he could start. If Pouncey starts and Center and Andre Gurode starts at (left) guard, Gurode could be the backup at Center, and the backup at Guard could be more of a pure guard. The same would be true if Pouncey won the left guard job; he could back up the center position, and a pure guard could come in to replace him.
2) If we don't take a tackle early (Rounds 1-3/traditional Day 1), McQuistan would seemingly be the backup at Tackle. That doesn't make me feel good if Doug Free or Marc Columbo go down. So I think that this would mean Leonard Davis is the backup Tackle, and a pure guard and a pure center could be the backups on gameday.
3) Procter is TERRIBLE. The team should do whatever is necessary to keep him off the field. I'd rather have Bigg at LT than Procter on the TEAM, let alone the field.