OL...OT specificially...starters established, depth needed...took Wesley Johnson, C 5/173...apparently this guy can play OT or C...not sure what to expect, but if Pouncey is not extended, he could be the starting C in 2015.
Johnson could also be seen as insurance for Adams. We already have 3 to 4 "swing" O-lineman in Pouncey, Wallace, Whimper & Beachum (Whimper & Beachum being the only OT capable/experienced). 8 O-lineman already under contract for 2015 so Johnson is a luxury at this point and provides needed competition. PIT only dresses 7 to 8 O-lineman each game. But like you've stated before though ATL, Gilbert may not be resigned after his contract is up. So Johnson could be seen as a necessity as well. As of right now, I think Johnson & Whimper are in a heads up battle for the 8th spot.
And don't forget about Embernasty! He is likely going to battle Wallace for the inside swing player.