I think its easier coding for them to have stoneskin take the dmg till it wears off, instead of having varying things which would act first instead of stoneskin. In a lot of mmos you need to remember "oh I have this on, its not useful to use this", plus most of the level 50 tanks I know who are actually tanking (you do not tank pre 50 w/e, the mobs die too fast), this is not a problem, they have already remembered redundancies and such. If this is really important, which is isn't, they'd probably most change it at 2.0