I'm not sure it warrants a dissertation, so I'll just say this: the MMO market in general has been attempting to move away from niche play—significantly varying outputs between classes wherein specific composition becomes essential or outright forced—in favor of similar outputs with varying styles, pace, or feel to their gameplay—such that classes feel different even without having to force a specific composition for each given fight. Needless to say, even then the variation given diminishes due to the .3% or so greater combined output of x combination when competing for best possible clear times, but at least it prevents classes from being entirely locked out of certain fights at the hardcore, midcore, or casual levels of progression, in ascending severity, as you'd see with intentionally niched design. Were SE to classify and distinguish the two casters as you're suggesting, there would be no choice in casters, or possibly even DPS, for certain fights. Summoners would be banned from the majority, save fights like A2S, while Black Mages would either be taken en masse or still skipped entirely because of their mobility issues in favor of a pure physical comp.
3.0 specifically tried to undo niche gameplay; this was their specific rationale to nerfing Holy and Flare (player disappointment when the two central AoE classes weren't present for a would-be speedrun because they were so damn strong). As usual with SE's implementation, or indeed any MMO's, they may have overshot the target when considering the sheer amount of AoE they've given Summoner. But the stated intention were balance, and not some oxymoronic balance of niches (which cannot exist so long as a party is able to change composition at any given time, because the fight-wide disadvantages of a given class can always be bypassed by replacing said class), but by attempting to better level output.