Matt Cassel may just be Scott Mitchell: a good backup quarterback that was helped by great coaching and good teammates to a good but brief run, which was enough to make him appeal to teams desperate for a quarterback.
But the Vikings must take a risk at the quarterback position--it has been a mess for Brad Childress's reign. After trading Daunte Culpepper, Childress has started the backup-level QB he inherited (Brad Johnson), the D 1-AA project he drafted (Tarvaris Jackson), and a series of cheap, bad backups (Kelly Holcomb, Brooks Bollinger, Gus Frerotte). The Vikings have just scraped by at quarterback without making a serious move to improve the position. The result: in 49 games in 06, 07, and 08, the Vikings have thrown for 47 touchdowns, 52 interceptions, and 183 yards per game. They've lacked the ability to sustain drives with the passing game, they've lacked a two minute offense, and they've lacked the ability to come from behind. You can (and should) put some blame on the Childress/Bevell offense, and you can (and should) put some blame on the quality of receivers. But a big reason the Viking passing game has been terrible is that they haven't had a good quarterback in three seasons.
It didn't have to be that way. In 2006, there were rumors they could have traded for Matt Schaub, but the price was too high. Schaub has dealt with injuries since joining the Texans, but in 2008 he averaged 276.6 yards per game, 8.0 yards per attempt, and posted a 92.7 rating. It would have been a risk. But quarterback is such an important position that a team can't compete for a championship by scraping around at backups that are barely replacement level.
I don't know if the Vikings should pursue Matt Cassel. But I know they have to do something significant. If they maintain the status quo, if they again acquire cheap, mostly undesirable quarterbacks and are forced to start them, they might compete for a division title. But we can't seriously expect the Vikings to compete for the Super Bowl if they don't make a major attempt to acquire a quality quarterback, whomever that might be. If the roster is full of backup quarterbacks that the team is forced to start, the team isn't going farther than it did last season.
There is a risk in acquiring a player like Cassel, just as there was a risk in acquiring a player like Schaub. But those are risks a team has to take to compete for a championship. If they take the risk, it might fail; if they don't do anything, they maintain the status quo, and I think that ceiling is 10-11 wins and a playoff loss.