When you have a scrambling QB, sometimes it can be a good idea to put a linebacker spy on him. Basically just designate someone to follow RGIII around the entire game, wherever he goes so goes the spy. With III being so versatile, and being able to outrun many DB's in the league, they'll have to be careful.
The downside is, you of course leave yourself a man short in coverage, run defense or the pass rush so it's a double edged sword. Should the Steelers do this then it would likely be Timmons because he has good quickness and speed for a big linebacker. He's not going to outrun III but taking a good angle could at least slow him down and limit his effectiveness on the ground.
I've never really been a fan of the spy necessarily, but Lebeau might use it in certain situations where the entire playbook is open. You wouldn't necessarily need it on 3rd and 15 because that's a more obvious passing situation but perhaps on 2nd down it could effective.