I believe that in AoE strat groups who try to max their clear time, they purposefully sacrifice BRD's DPS to sing Ballad so WHM can sustain the Holy spam, regular DPS, and keeping up solo heal, so you might be able to do some pet swapping in such a group. I could be wrong about those groups using Ballad, but from my experience the main limiting factor of a WHM DPSing above 100+ while solo healing is MP unless they do some high level play where SCH puts up DoTs then will take over healing while WHM DPSes until DoTs get put back up.