In the end it comes down to this. What are RDM's primary jobs in a endgame party?

It is to deal damage and to provide support for the party in terms of getting dead ppl up quickly to minimize downtime and dps loss.

Could RDM do that in a long EX/Savage fight with the old system? The answer is no they could not. This is why the mp was changed.

rdm's dps is low-mid tier, if they cannot raise or spot emergency heal while maintaining constant dps... They are a completely worthless party spot.

In ex innocence and titania yes I could do it without stopping dps when using LD on cooldown you are right, I would almost run out but LD usually came back up when I was around 1500-2500mp left... but I could not perform any of my other job responsibilities. Literally had to let the healers hard cast raise on dead ppl cause I did not have the mp to instant raise them.

Once the group is solid enough to not mess up mechanics yea does not matter... but at that point well you are better off switching to blm or smn anyways.