The self defense at my school is an amalgamation of aikido, jujutsu, hapkido, judo, and a little wrestling. We also incorporate weapons from Japanese Kobudo. However, the style we practice is not a 'pure' style of TKD due to the influence of our Master's training in karate and aikido, thus we also have techniques taken from those.
I suppose my personal take on this topic is that there are some styles which philosophically do not go well together and thus to incorporate one into another, some changes in the philosophy must be addressed. (Aikido's soft flowing avoidance vs. Tae Kwon Do's hard blocks and direct strikes, for example). Also it was brought up that when the old masters incorporated new elements, they often changed the name of the style, but that begs the question: At what point does it become a new style and when is it just TKD with some of (insert style here)?