I challenge the idea that learning a different role won't do anything for anyone other than that given role.

Scenario:
DRK, BLM, MCH, WHM, Tank dies, WHM's Swiftcast is on CD. BLM was 2nd on hate and is positioned next to the healer.

BLM: They're coming for me. The tank had them gathered around the MCH's Bishop turret. I should move into the tank's position until the WHM can get him up. I still have Stoneskin, but that won't last long. Should I Manawall? The one the tank was attacking is nearly dead and going for the healer, but it's not so big a threat. The MCH can finish it off, let me get the rest.

MCH: Tank's down and they're getting away from my turret. One is nearly dead, I should be fine to DoT and Blank it away from the healer. BLM's gotta pseudo tank now. Let me Hypercharge and lend some AoE support. Tank's also gonna need some TP when he's up. He's a DRK so he ought to be able to handle his own MP, but I'll keep an eye out and switch when I have to.

WHM: Dammit, bad time to Swiftcast Holy. I didn't see he used Blood Price so he ran out of MP and couldn't mitigate as well. The BLM has hate, good he's moving them away from me back to the turret. And the MCH just finished off the weak one. Let me put down an Asylum, give the BLM a Regen, and Raise the tank.

^Pretty extreme scenario, but if they were ignorant of each other's roles/abilities, that situation would be a guaranteed wipe. With that awareness however, they can at least attempt to control the situation.