I have seen a horse's legs get too dependent on boots, BUT the owner was scary religous about their use, not allowing the horse out of its stall without the boots on.
I think they're fine in moderation.
The best way to "preserve" the horse's legs is to work the horse responsibly, slowly building up. Work with and without boots on all kinds of surfaces, conditioning the horse over time.
Personally, I only use bell boots if a horse overreaches, otherwise none of my horses are booted. We do trails (some road riding), Dressage, jumping, and basic western. We have horses from 5-14 yrs old. The older guy may get some support boots as he gets older, but he works fine for now. I give lessons and the horses are worked often, 4-6 days a week, 1-3 lessons a day.