I might be remembering this incorrectly, but as I recall Coleman Shelton the Center was solid. It was Coleman Shelton the guard that was below average. Sure he could get bull rushed by some bigger DTs from time to time in one on one matchups but so can most centers. I believe that Poona Ford destroyed anyone the Rams lined up at center (Sullivan, Allen, Shelton) when he was in Seattle.
I guess the key in team building is team. It's impossible to have 22 studs in the staring lineup. And if the Rams believe that Shelton's experience, fit, knowledge of the schemes, consistent availability, and relationship/communication with Stafford are what is best for the team, I have no problem with his signing. Especially when it's just $12 million for 2 years.