This could turn out good or bad for the Eagles.
They will be very versatile on offense, and able to give defenses a lot of different looks. Teams will have to study two different very potent QBs instead of just one. This is a lot of extra work. There will be a running threat on the field besides Westbrook. Also, now that McNabb has a legitimate backup, he'll feel the pressure a bit more and might perform better because of it. Also, McNabb is old; If Vick works out, they get him on a long-term and he becomes their QB for the future.
Downside includes of course the ramifications if Vick does not perform. Also, the distraction to the team that he will inevitably bring. Also, him and McNabb are completely different QBs. How will the offense adapt on the fly to these two different styles?
In my opinion, the upside trumps the downside at this point considering his low cost.