You make a good point here. While there are obviously many health benefits attached to leg training, the truth of the matter is that leg training (when done for the purpose of growth) will cause one's caloric requirmements to rise more quickly...by virtue of the fact that they will be carrying more overall muscle tissue.
This means more total calories will be needed just to maintain muscle tissue, let alone gain more, forcing the individual to consume a larger amount of caloroies to fuel the growth of other bodyparts.
For example, if I were to gain 25 pounds of additional leg mass, taking me from 250 bs to 275 lbs, my caloric requirments--from a mainetance standpoint alone--would increase by a good 1,000-1,500 per day. In other words, I would need to eat an extra 1,000-1,500 caloroies just to maintain the same amount of chest, arm, back and delt mass I had at 250 lbs...and if I wanted to gain additional upper-body mass, I would now be required to eat even more calories just to make that growth possible.
Now, for bodybuilders who want to maximize their development/compete, this is just part of the game. If they stop playing, they stop progressing, but what about those people who don't have competitive aspirations and/or are already content with their leg size? Should they be forced to build additional leg mass--thereby drastically increasing their caloric requirments--just so they can continue adding muscle to their upper-body? Of course not.
So, while the notion of leg training may seem like an automatic prerequisite for serious bodybuilders, we should also understand that not everyone has the same goals.
At a minimum, I think it would be wise to at least train legs for mainetance and general overall health purposes, but there is no rule that says those who want to continue building muscle/strength in other areas must continue growing their legs as well.
Often, as bodybuilders, we have been pre-programmed to think that those who don't train legs hard, or don't train them at all, do so simply because they are lazy, undisciplined, weak-willed or have a poor work ethic. In the case of a professing, aspiring competitive bodybuilder, this may very well be the case, but this is not so for everyone else. Some people may indeed just be lazy, but many people just don't have the desire to grow their legs anymore.
In the end, to each their own, but I would still recommend including at least some maintanance leg training for health purposes.
You hit on something that I do not think should be overlooked "overall health". I"m intrigued by the studies about leg strength and leg work and overall health. It's also great for bone health and something any aging person should include.
But it doesn't have to be a ball busting leg workout, as you noted. I mean hell throw in 3 sets of 10 with 135-225 on squats twice a week and call it a day.