Admittedly, I haven't done any high end content, but the "weird notion" of 2 melee 2 ranged seems to be a biproduct of both strategies being designed for that for balance, combined with the fact that you get main stat buffs by having Tank, Healer, Melee, Ranged Physical, and Ranged Magical in the group. That covers the 2 ranged for the bonus (1 physical, 1 magical) then you have an extra spot which just defaults to melee because of symmetry it seems.
As for the healers, typical they want 1 shield and 1 regen healer which, again, is not "weird." It's because both are good in certain situations and having 1 of each creates a powerful team (2 regen isn't as bad as 2 shield, but it still isn't as powerful as 1 of each, that's why there's "no doubles"). Fact of the matter is the game pushes people toward this "weird" notions by how it's designed, sometimes it does it explicitly (e.g. the main stat bonus for one of each job type) and sometimes implicitly (e.g. not allowing shields to stack, regens and shields being better in different situations, giving classes different kinds of group wide damage buffs), but it's the game that causes this, it's not some weird superficial rule that the community has come up with.
Also, the community doesn't do this _now_ because of how things are set up. Though I'd be surprised if people during progression or people competing for top kill times don't grab specific classes if all other things are equal (e.g. player skill). RDM/SMN for prog groups because of the extra rezzes for example. BLM/SAM with some good group buffs if they're looking for fast kill time, etc.