It comes down to WHM being better at burst DPS and SCH being better at sustained. SCH have more DoTs, better MP management and also a pet that can heal while they DPS. In longer fights that will always be preferred as the main DPSer because they simply have the better tools to do it. As a WHM, you can still look for opportunities to DPS, but only if you can fulfill your healing requirements first.

A2S is a great example where WHM DPS shines.