After thinking more about this signing, here are a couple of my thoughts:
1) Although there are better veteran tackle options still out there, it's possible they are wanting to sign somewhere where they stand a chance of winning the starting job or want more money than Dallas believes is reasonable for a swing tackle.
2) Erving is possibly more of a fallback if Hyatt, Knight or Steele fail to seize the spot or if Williams ends up starting at left guard. Erving's not a great tackle, but there's always a chance the others could be worse.
It might be in Dallas' best long-term interest for McGovern to prove better at left guard, so that we can find out if Williams can ultimately take over for Smith at left tackle. Line in a year could be Williams, McGovern, Biadasz, Martin and Collins. Or course, we need to find out if Williams, McGovern and Biadasz can handle those roles.