I believe that MRA's are essential to a world. This is proven by how every world has and will forever have large MRA's.
They offer a common enemy (BP target) and a chance for inexperienced players to grow and learn the game.
Ever hear of a completely "elite" world? That is because the world would not be like that without a MRA.
Every world will have MRA's because every world will have new leaders that dont know any better and see having lots of members as an example of how strong they are. Of course as we know a true reflection of strength has nothing to do with how many members or points you have
choose a small elite alliance and end up with 10-15 players out of 40, then try and recruit or become too elite. choose a big alliance and get stuck with boring internal conquers, ghost cities and teaching as a second job.
the fact that mra's have to defend a lot is not surprising since they often accept nearly anyone, then have to support them. the fact that they get targeted by multiple alliances at once makes the elite alliances who team up with other not that much different in size.
in a perfect world without inactivity, somewhere between nat and nr in size would be ideal.