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I don't believe there are diminishing returns in this game.
The more attack you have, the more you are above enemy defence so each new point is actually worth more than older. (if that makes sense)
As for being fragile. Are you playing on hard? I believe you will be fragile no matter what on that difficulty.
Only "diminishing return" i can think of is fun factor you mentioned. As long as you dont intend to break records in killing Online UR you won't notice much difference i think.
I'm little ocd so i made 3 characters, 550/550 atk/mag, full atk and full mag. Ended up laying 550/550 because its effective no matter what vocation i feel like playing. And really dont see much difference from other two, unless i would try playing sorc on full atk char ;)