Sage Rosenfels is one of the better backups in the game, but that is all he is, a backup. If you look at how he has performed as a starter over an extended period you will see he just can't get the job done, which is why he never held the starting gig in Miami or Houston.
Personally, if we sign a backup, I want someone who can start and be effective over an extended amount of time due to Trent's injury history.