Agreed. We need a system like that, not account wide ones.
Account wide makes no sense, because what's going to keep me from leveling up 4 characters, doing easy quests, and reaping the rewards so much faster? Now if you want a system where your highest rep is the one that carries over and all of your alts don't generate rep, then maybe that be okay, but I can't imagine the coding that would have to go into a system like that.
A system where, for every alt, you generate 50% more rep, maybe even 100% more if you're exalted on another character, makes a hell of a lot more sense, and gives more than enough to cover the time investment of grinding on your main, while still having your alts feel like alts, not just what you're asking for, which is essentially a spec switch. Our characters are no longer unique when we can just hop in and out of them and retain nearly all of the rewards we've already gained on another character.