Its useful if you want to level up faster and don't care as much about the other skills. In my case, I'm planning to use it on my newer game (that will be a "base" before doing quests that require choosing a side).
That way, when I kill any future bosses, I'll get extra XP from it, and from any monsters in the future. I also plan to grind for the L&E items (I was in the gargoyle cave but got bored) and I'd like to get more XP for my efforts in each area.