If you ask me the key point to think about is how long has the backup been around for. If the backup is relatively new and hasn't started games, those are potential disasters. If it is a guy that has had a few years in town, time to learn and adapt to the system, then those are generally the good bets.
A guy like Simms right now is a tweener. He has been around to learn the system a whole year, but had no playing time until this season and still probably needs this whole season to be able to succeed next year.
If the starter was Koy Detmer, that would be a spot I would love to back the Eagles in. Taking a guy that has been around for years in a fairly structured system and who has had starts before is generally less of a reduction in price than the line will reflect.
For another example, if Warner gets hurt, I would stay far away from Eli Manning. No experience and not enough time in an NFL system, plus the public thinks he is as good as Warner so the line wouldn't adjust much. Those are the spots you might fade a guy.