When does hybrid xp penalty get removed?

[Dulu]

I'm just curious, but what exactly was the rationale, if there ever was one, as to why the XP penalty was implemented on certain classes?

Casters.. 10% Flat xp penalty? Why? Don't they kinda have it hard-enough with having to sit on the ground 85% of the time, pay for spells, find said spells, etc?

I understand the racial penalties, but I think those could have been handled better too. Instead of terrible XP penalties, maybe just further reduce their dexterity and agility. Seems pretty straightforward, but I've never understood the XP penalty system.