I don't think boxing is under used or rated in mma at all.
Many (if not most) mma fighters have boxing coaches and had the striker skills and footwork of boxing to their game.
GSP the most notable recent addition, working with Freddy Roach (not to mention BElfort, Silva and Rampage all working hard on boxing)
The thing is only elements of the boxing discipline can be effectively used in an mma fight so I assume when people train their boxng skills they do so in a fashion that suits the type of fighting they will be undertaking, so it may not be overtly obvious when you watch a fight that someone is applying their boxing skills.
But this does not mean they are not and have not improved their punching techniques via boxing training because they have to also focus on factors such as low kicks and take downs all of which are alien to a pure boxer.