As far as cost goes... Cassel will make $3.7MM this year according to NFL.com while Freeman will earn $3MM according to Yahoo.com. So, that point is even worth considering because they will essentially earn the same amount of money.
Lastly, ultimately, Freeman signed a one year deal with the VIkings so that alleviates your final concern about Freeman being stuck in purple while he stinks up the joint like Cassel & Ponder.
I imagine that you are completely on board now, right?
For me, at the end of the day, it is obvious that Cassel and Ponder aren't good enough to lead this team back to the playoffs. Signing Freeman is a risk to be sure but it is a risk worth taking considering the deal that brought Freeman to Minnesota. One year for $3MM is fine by me and let's see what he can do when defenses stack the box to slow down Peterson. If he sucks, then he's gone at the conclusion of this season and the Vikings begin their search for a new QB in the draft. If he's great, awesome. I really fail to see the downside.