aDPS is an important metric to look at.

If we only had rDPS we wouldn't understand how jobs like SAM or BLM truly benefits from raidbuff.
rDPS is a much better metric to understand what a job provides to the group.
It's important to not only focus on one but to understand both.

As for "MCH outperforms BRD/DNC in other content" that's not true either, but more importantly that's not relevant.
BRD is a big winner in AoE games. It makes sense that DNC falls off when a lot of players simply don't care about doing good DPS in content other than savage.
In dungeons, 24 mans raid and other, you could have massive discrepancy it wouldn't matter.

Savage and Ultimate are the only places were DPS matters, therefore the basis for DPS balance.