Legursky and Foster were tendered -- this is not earth shattering news. This is merely the completion of a formality. Legursky has value as a backup G/C, and I'm comfortable with him starting at C if Pouncey gets hurt. Foster can play both guard spots adequately, though I prefer him as a backup, not a starter. Essex is a very solid 9th/10th guy on the offensive line. He can play any position with at least reasonable competence, and as the season goes on, it never hurts to have a player like that.
Ideally, the Steelers add at least one premium offensive lineman in the draft, whether at tackle or guard, and send one of Foster or Legursky to the bench. Having both in a reserve role would be ideal, but if the Steelers add one premium player, I think you can get away with Foster as one of the starters. If Foster and Legursky are starting at the end of the season for some reason other than injuries, things have not gone well.