I agree that using atari go (a.k.a. capture go) as a stepping stone to teach real go is redundant at best, but I know that many go teachers do it, so each their own I guess.
The main issue I foresee is that beginners starting with stone counting on OGS and moving on to area counting might keep filling in their own territories before passing. If they do that under area scoring rules or territory scoring rules, they could get reported for stalling.
To me also it would be just a gimmick. But OGS seems not averse to adding gimmicks (for example Feature Request - Stone Images).