In all honesty? I doubt very few use most of them. In theory, a +10 Level 4 weapon would kick lots of awesome, but considering how hard it would be to pull that off, it's not really worth it.
The trend is generally using a +10 level one item with as many slots as you could get [ Generally 4, though books only give three, and I'm not sure about a any other weapons that are restricted like that. ] and stacking it with high powered cads. While Gravity might have hoped to make level 4 weapons useful, they're not used that often, are they? xD
I often do the various quests for the sake of doing them. Doing something different to get a feel for the game's less commonly noticed features is about the only reason I've ever done any of the non job change, non dungeon-access related quests.