Its about balance to be fair, the alt run has never been a boost group, we've got away with it in normal because Normal is easier than LFR.
What became apparent the other night was that some fights, have a dps requirement. We've not noticed in main raid as we were already on the point of outgearing it. Kingaroth for example, I've only seen 4 waves of adds our first week, we weren't looking at him being dead before a 5th. Eonar as well becomes problematic without decent burst. Same for Hasabel, Adds on High Command, only so much can be offset by the higher DPS alts.
What this doesn't mean is that people are arbitrarily banned, Each week when we start Heroic, All DPS, that the healers are capable of supporting will be allowed to come, Worldbreaker will be the bench mark against which i'll filter the raid for later fights. I will probably extend this to include Hounds as well. So nobody barred, everyone will get the opportunity each week to show they've improved and got themselves upto the required standard.
To Best Buddy, go to Wizie, oh wait, We don't have any Arcane Mages you can ask for advice.
Damn Witch's DH has done fine most runs she's come on.
Thermo would usually come as Heal, Scarce, I rarely say no to more healing, BM Hunters however, they get all whiny when I send them into Portals.