AbdulAlhazred
Legend
Allowing the save bonus to be derived from one of two stats was a heretical crutch to allow better defenses for some classes. It would be a compounding of sins against the Holy 6 Ability Scores to allow skills to use one of two stats.
Its more like having 6 kinds of defense is too much of a good thing. 3 is enough. Not that the resulting system is perfect, but I think that was original thinking behind that game mechanic. 4e actually just inherited it from 3e and you notice that in 3e those were saves and thus active. 4e regularized things so all defenses were passive and that made them distinct from skills, thus we end up with the Athletics, Acrobatics, Endurance, etc skills, which could all really simply be called attribute checks. Then the issue would be training, so you'd still have to have feats to allow those checks to be boosted, or else you end up with 2 parallel systems.
I agree, endurance is kind of a weak skill, but it was a logically necessary one. Also it isn't as weak as people make it out to be. Endurance checks don't come up a LOT but when they do, they are usually pretty important. Its one of those skills you won't need often, but when you do it will save your skin (at least in my campaign).