You would be giving and unfair advantage to the team with the Mentor IF something like this actually was implemented. The mentor could talk strategy with his team while the other team is forced to use quick chat. That does not mean that the Mentor team is an automatic win since it all boils down to skill, but you're giving the team a +1 already with unrestricted chat. Both teams need to be on an even playing field.

As others have mentioned, Mentor status comes from completing a vast amount of PVE criteria. That does not translate into PVP since it is a different monster compared to PVE.