Here is my thinking.
The Giants kept 3 QBs on the roster (unusual for them) because they didn't want to expose Nassib to waivers, but they didn't think he was ready yet. The thought process was that if Eli was knocked out for a stretch of time, Painter would give them a shot at winning half the games.
Does anyone still believe that?
With as many injuries as the Giants have, couldn't they use that roster spot? Why not use it to check out a young offensive lineman or a linebacker?, or even add another running back?
If Eli is knocked out for more than one game, they can make a call to David Carr, who is still hanging on a couch.