That is not entirely true. Most people would prefer the option that pushes the most DPS per fight, that would determine the classes that are brought. This generally influences party make-ups as a whole when strict DPS checks are thrown into the mix. Those that can push the bigger numbers with less restrictions will be the main stream comp (DRG situation? Pre -Buff). The only time you generally see things like one melee, two casters and a bard, is when you are forced into these situations via whose available for your static or some other unorthodox method. Very rare are they're fights that heavily favor AoE aspects, which the Caster classes excel at, because it creates terrible imbalance vs the Melee. I do believe Turn 4 alone was the last time we saw something like this.
The only reason people keep casters around in truth? Because of a Caster LB /AoE control, should it have use, otherwise since single target DPS is a heavy thing for most boss fights, classes that excel at single target DPS tend to be preferred.
Melee do not have the same resource troubles in it's entirety either. Invigorate does not hamper your DPS, but rather gives you FREE TP per CD. Energy Drain, DIRECTLY counters your DPS option of using Fester / Bane instead because it's tied into Aetherflow Stacks (you only get 3 / min).
