I spent a lot of time learning Taiji, and trying to figure out how I might use it to fight. The problem I have had always had is that (1) those that practice Taiji for the most part had no interest in fighting with it or had (like some people on this forum) unrealistic ideas of what combat would be like (imaginary qi-powers, etc.), (2) it's often (not always) practiced far too slow despite what the Taiji Classics say about "beating people to the punch" to paraphrase it roughly.
I think it's possible to use for fighting, but there are more efficient ways to do so, and more effective. That being said, if you like doing Taiji, then practice it, no one says you shouldn't, but be honest with yourself about what you can or can't do.
Another thing I might add, Taiji principles are actually pretty solid. The problem comes about when people try to format the principles according to their own movements, and not their movements according to the principles.