Why train legs? As an Olympic lifter this sounds like a ridiculous question to me, because that's just about all I do. However, when met with this question, I couldn't find any non aesthetics based answers. If aesthetics are not the main goal and particularly strong legs don't have direct carry over to a sport, what is the argument for training legs? Male gymnasts for instance have to do lots of holds on rings and upper body type exercises where beefy legs would be a detriment to their performance. Conversely cyclists aim for as small an upper body as possible to drop weight for climbs etc.
Is there a health argument for having an upper/lower balance in training?
Not necessarily a performance benefit because that's pretty clear, asking mostly for the laymen here. If you're happy with your looks and don't feel like training legs because you like how they look, what arguments might make you reconsider? And same goes for arms. I see many women not wanna do arms because they are fine with the look of their arms, or cyclists or people in any other circumstance (not including medical problems in this) that'd make them not want to train a certain body party.