Indeed, we have already heard clamoring from some that the Vikings should go after Kirk Cousins, who played well in a limited relief role for Washington last season while it dealt with RGIII's injury.
The first preference, obviously, is that Christian Ponder breaks through and becomes at least a middle-of-the-pack starting QB in the NFL. If not, though, we would have no problem with the Vikings starting over with a rookie in 2014 if that's what it takes. It wouldn't even be crazy to go after the right veteran if the team felt, as it did in 2009, it was a QB away from a serious title push.
But making a move for a backup who has starred in limited action? Stay as far away as possible.
For a good example, look at Matt Flynn. He signed a deal with $10 million guaranteed for Seattle after a couple of nice games in Green Bay, couldn't beat out Russell Wilson in Seattle and might suffer the same fate in Oakland to Terrelle Pryor. Matt Cassel got a $62 million contract in Kansas City after one good year in New England, and the Patriots also got a high second-round pick in the trade that sent him there. Now Cassel is 31 and waiting in line behind Ponder.
Once in a while, you might find a solid player going this route (Matt Schaub, acquired by Houston from Atlanta, has at least proven to be an upper-half QB), but overall: these guys are backups for a reason, even if they are sitting behind elite NFL QBs on the bench.