It depends. All tanks have AoE, and ranged and casters have AoE available if synced. The only time it might not be a good idea is if both of your dps are melee and/or 15-17 casters/ranged, but even then the tank's AoE is strong enough on its own to make it worth doing if the tank is the only one with AoE. For Sastasha and Tam Tara specifically, I usually strike a balance and do 2 groups at a time so as not to traumatize any newbies, particularly the healer.