That was mostly true when 2.0 was launched, as there was very little AoE damage and SCH excelled at 99% of the fights in the game (even coil 1-5 had mechanics that favored SCH/SCH instead of WHM/SCH).This lead to a huge discrepancy in overall healing effectiveness, as SCH also had the benefit of bringing unique buffs/debuffs to parties that WHM don't have. Basically it became a question of "If both jobs can do it, but one has better utility and no mana issues, it'd be better to go with that one."
You can tell SE took notice to the increasing amount of SCH/SCH parties. Once Labyrinth of the Ancients hit, we started seeing a ton more AoE damage skills coming from enemies. Syrcus Tower and 2nd Coil expanded on this even further, and now there are some really hard hitting AoE abilities that purposely play to WHM's strengths.
When 2.0 launched WHM was really only good for pugs and DF, as a coordinated party would make better use out of a SCH/SCH pairing in end-game. As of 2.4 SCH/WHM pairing is the more efficient option for end-game, and I hope SE keeps that balance (at least until 3.0 when we see the next healing job).