If he makes the roster he will have a chance to back up Ryan Clark. As you saw in the Super bowl he isn't much in pass coverage, but is terrific in run support and a great leader. If Greenwood makes the team Clark would be a good guy to learn from. The Steelers run a complex defense so you need a smart guy at the FS spot which benefits Greenwood.
Greenwood is in a similar boat as Tarp your way to making a roster is by special teams or making the practice squad.