I'd much rather have the choice of fighting for a doomed cause than having none at all.because if they weren't they would be in the small fanatical group that is made up only of orcs (who are likely almost all going to die), some gallywix loyal goblins, and enslaved members of other races. garrosh is a complete failure when it comes to tactics, his plans are all extremely basic and dependent on a monster or death trap being pulled out of his hat, and if the people who love him haven't noticed, HE'S BEEN LOSING THE FIGHT SINCE HE STARTED IT. the barrens? alliance took ground in the barrens it wouldn't even have if he hadn't attacked them, every move he made in pandaria was countered or demolished, he can't even scratch lion's landing while alliance forces are actively invading domination point, gilneas a win for garrosh? HA! all of that was done by sylvanas and she wouldn't have even fought the worgen there if not for garrosh forcing her to, which is part of what brought them into the alliance.
garrosh is only good for starting a fight and getting his troops killed but you want to join his side? fine, roll orc, go to timeless isles, turn on the censer and see how long you last...that's your authentic garrosh supporter experience.