I believe that it goes without saying that many of us don’t play on maps specificaly on our level - at least when it comes to questing.
My Wizard was at level 73, done all the quests she’s had, and was questing at the Mausoleum’s 1st Floor - a Level 81 map.
This 8 level difference would only further be troublesome since not only mosters wouldn’t give me as much exp (even if I was able to kill them well enough), but especially bosses.
This basically makes Exp Cards the only source of reliable experience, since Grinding takes too long and franky, I’m more of a quest player.
I think that if the exp penalty range was 10 levels instead of 5, it wouldn’t really lock out players from having to grind - it kind of makes other players feel guilty about questing in a map where they know they’re actively getting less exp because they don’t have a higher level.