For any goal, there's only gone to be one (or a best a couple) optimal builds. Among players, the vast majority are going to have one or the same few goals.
In a MMO, most players are going to be chasing the most overpowered build. Which is why MMO developers change things all the time - not for supposed "balance", just to keep players on the treadmill.
I agree on the MMO part. That's why for me MMOs aren't really RPGs. My statement about good RPGs was a little different, though. A good RPG really has different optimal builds, e.g., you can't say that one of the starting characters in Diablo 2 is really better than the others. If one build overpowers the others that's really a balancing issue and a game claiming to be a good RPG has to make sure that this doesn't exist. As you say I also believe that MMOs have the goal to keep players on the treadmill and therefore prefer a changing imbalance over a constant balance.
In a MMO, most players are going to be chasing the most overpowered build. Which is why MMO developers change things all the time - not for supposed "balance", just to keep players on the treadmill.