I think those who question having a veteran mentor for Mark Sanchez are a bit off base. The idea itself is not a bad one. I know the Jets have coaches to guide Sanchez, but having another experienced voice who knows what he is going through and can spot things on the field can only be helpful. Brunell is a natural fit for this. He saw an awful lot during his time starting with the Jaguars and Redskins.
The most important role of the backup quarterback, though, is the ability to play football well. The backup is one hit away from being the most important guy on the team. In this area, Brunell is not a very attractive option. I know he threw a couple of touchdown passes Week 17 against the Bills. He also threw a lot of ducks in that game. His arm strength is virtually gone. There are a ton of throws he cannot make. He also does not have pinpoint accuracy like Chad Pennington to compensate.
A mentor is great, but that is not worth risking having a subpar guy starting a Playoff game. In an ideal world, a backup quarterback would both be able to mentor Sanchez and play well. The ability to play well is much a more important quality, though.