In my opinion, Taiji can be applied in a myriad of ways on multiple levels of self-defense. In the world we inhabit today, it is far more useful in terms of self-defense against stress, disease, and orthopaedic problems than it could possibly be for fighting. However, in my opinion the majority of these benefits are so relevant to practicing the art as a fighting style that the two are relatively inseperable with the later being a motivator through which the former may be achieved as well.
I also believe that true self-defense has been a concept with realitivity little to do with actually fighting throughout the history of un-armed martial arts in particular. For example, even a millenium ago before projectile weapons were the main constituent of personal destruction, individual bravery and even individual skill had much less relevance to warfare than did the overall prowess of the group. Even then, personal combat was likely to be dominated by the use of weaponry and group tactics. I believe a quote from Douglas Wiles' book "Tai Chi's Ancestors: the Making of an Internal Martial Art" summates the relevance of individual combat ability (or self-defense as we like to refer to it) well when he qutoes a famous Chinese general as referring to skilled martial artists as "useless on the battlefield." The fact that we fight in groups and rely on technology is not a new concept, and neither is the concept of fighting ability having much less relevance to personal self-defense than other forms of self-protection.
In conclusion, if your question is "can you use Taiji for self-defense" the answer of anyone who can improve their well-being should be a unanimous "yes!" as the side-effects of training and their protective attributes in other fields is, in fact, the most important feature they provide.
"If you don't eat the bitter, not only the bitter but the most bitter of the bitter, you will never be the best of the best."