I suspect that very few people train for only one element. Though it may be a single element that drew them to the class, it is likely multiple elements that keep them training over the long haul. Many who train for "self defense" probably reach a point where they can adequately defend themselves within a relatively short time (less than four years, and depending upon class structure, personal attendance, and outside practice, maybe less than one year).
Many train for that black belt with the idea that once they have that, they can "defend themselves," when in reality, the belt is psychological; they probably have learned the skills to adequately defend themselves long before the black belt, but that belt tends to make things click mentally.
I got into taekwondo for self defense as a skinny, uncoordinated child (that Jhoon Rhee commercial got me started in Kensington, MD) in order to deal with schoolyard and neighborhood bullies.
However, at this point, I practice the forms for fitness. My main motivation (versus the practical elements of fitness or defense) is simply the joy of learning new things, increasing my depth of knowledge, and simple pleasure. I don't currently train in a dojang and I am not currently teaching taekwondo (both of those may change at some point in the future). Self defense is no longer a factor in my personal training motivations, nor is rank; I hold a fourth dan from an independent school and a KKW first dan. The belts don't get any blacker at this point.
Now, I'm just into the arts; I'm no longer underweight, I'm quite coordinated, and I've been more than able to defend myself on several occasions over the years, and in every case, with the most rudimentary of taekwondo skills (all things I was well practiced in by green belt). In fact, to date, I have never had to use any hapkido, Bartitsu, kendo, kenjutsu, taijutsu, or -any other art I've trained in for any length of time- skill in real world self defense.
The only skill I've used in a real world application from hapkido has been break-falling, and that was only once when I slid on black ice in a parking lot at night.
I use the non physical skills those arts, including taekwondo, all the time in order to avoid/resolve situations before the physical skills are ever needed. But those are all skills that I first learnt in taekwondo, though they are not unique/specific to taekwondo.
