Well, I hadn't realised that Tai Chi was a martial art till I started it a month ago, and started doing some reading. So at the moment I'm in it for fitness, stress, those sorts of things, but who knows in the future?
My Sifu also teaches kobu (my kids both train with him), and prefers his Tai Chi instructors to, as well. This, he says, gives them an understanding of the martial aspects of Tai Chi.
Sifu always points out some of the martial aspects- eg, this hand is here for protection, or could strike from here etc. He's also always emphasising the power of balance and rooting, as if responding to an assailant.
Galante's book (which Sifu recommends) shows the martial arts side of just about every movement.
(But at the moment I'm concentrating on trying not to trip over my own feet ......!)