little risk? have you ever played a mage? (on sloth - this is sloth, not d&d, you do realize, right?)
1 - mirrors don't always block hits (i have had 4 mirrors up and taken full damage from a mob before)
2 - th prime stab at avatar is 24-25 dam, add in poison and th prime multiplier and th prime stabdam eq and you are looking at insanely high damage. 20 dam or so for a mage at high avatar levels is pretty good, plus no poisons and weaker stab and no stabdam and etc - you really think mirrors are gonna last all that long considering we are fighting same mob for at least 1 extra round? it just balances out the thieves much higher hps
3 - I'm all for game balance - adding a totally unrelated penalty to the only usefull prime skill of a class does not do anything for game balance, in my opinion
you seem to be highly overrating mirrors. they are nice, but do not give you 'free xp' by any means - spelldam does, sure, but if i am running mage xp mob (say chickling), if i do not kill in 1 firewind, i lose 2-3 mirrors and 1-200 hps. i then spend 40-60 mana to recast those mirrors, every single time. how is that free? and in the times i kill mobs with firewinds, that is spelldam and av bonuses giving me free xp - mob would die with or without mirrors, they just enable to run at +5 ac and survive