I think the reason the developers are saying the Manathirst MP regen is okay is because they know the BIG PICTURE. We will not be level 50 forever. Right now, I don't care about Manathirst because, quite frankly MP is not a big issue for me. So it could be a 25% refresh rate and I'd still choose upping my cures or base nuke damage over my MP pool. Afterall, it's not the size of the pool, but what you do with it.

However, around level 70, this might be a whole different story. We as players just don't know everything that the devs know when it comes to balancing out the game/classes/jobs for the future.