The support offered by Bards has traditionally been much more 'instrumental' (heh) to the raid than the support abilities offered by any other DPS class. Disembowel, Dragon Kick, Goad, Trick Attack, Battle Res/Virus from Summoners, Apocatastasis, and so on... these were all nice to have, but none were specifically required to progress from T1-13. However, I would say that pretty much every group had to take a Bard along. Until people got over geared for each floor, Ballad and Paeon were mandatory.

Merely having such powerful support options available is reason enough for BRDs and MCHs to be weaker than the dedicated DPS classes. If they could do the same DPS as a BLM by default, but offer much more support when its called for, then why bring a BLM?

When Alex Savage comes out, I think its safe to say that TP & MP regen will be necessary again, and every raid group will want to bring a Bard or Machinist along.