I think the XP penalties were put in place to punish players for being hybrids because of their spellcasting ability and flat out survivability (wearing plate and casting spells for three of the four -40%). I think monks were punished for roughly the same reason (mend and feign death). Casters have the penalty, but not as severely because of their innate squishiness...
Not that I agree with the original developer's decisions on this... If I remember correctly, at least one of the devs was openly hostile toward hybrids.
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Project 1999:
Tpyo - Iksar Necromancer
Xegony:
Saezuru Matsudai - 65 Wood Elf Bard

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