It's not about having the goods as a backup. Backups aren't franchise QB's, so the key to a backup is being able to pick up the slack until the starting quarterback is able to return. Yeah, sometimes players start as backups and have pretty decent careers, but let's be honest, Devlin is likely a career backup. Doesn't mean he can't be a pretty good backup, he just isn't anything spectacular. Like hoops said, Tannehill is the only one I expect to play quarterback in 2013.
If anything, draft a QB in the later rounds and let the young bucks compete for the backup spot. Devlin should know the offense and personnel well enough to secure a backup spot, though.