Still doesn't change the fact that random kills, levequests FATEs, and dungeons get stale and boring.
Some people might say that they didn't understand a movie until the watched it more than once...same can be said here.
Once the story is complete and your able to attribute people, places, names, and so on, you can better understand what you missed the first time.
(thats not the point though)
The point is, anything other than quests can get old very quickly even with a +50% exp boost.

Think of it as the games natural "replay-ability".

I played through FF7 several times because I enjoyed revisiting my favorite places, hearing my favorite music in the series, seeing my favorite characters and so on.
The main story quest is something you can only experience once per character while everything else respawns, re-ups, or can ultimately be played again right aftering completing it (with the exception of tribe quests).