There is no negatives for account wide rep. The grinds will be the same. The time investment to get to exalted is the same. I'm not going to suddenly have a 560+ ilvl when I ding 90. I will, however, retain the effort I put into the game when I change mains for things like rep achievements and rewards.
The account wide rep turning into an "alts require no work" is a slippery slope fallacy.