I think that dispel magic is the most powerful debuffing spell available in the game, and as such it should not be on the list of impairment spells.
Dispel magic, unlike any other debuff, can cancel out multiple positive buffs with a single cast. It also can be cast across the ethereal plane. If the person being dispelled has giant strength or haste, then dispelling those is actually a debuff in addition to losing those spells, since the affected person takes fatigue. I cannot think of many situations where I would rather cast slow, weakness, or poison instead of dispel magic in PvP unless I was fighting against an unbuffed newb, since warrior classes in general have much higher fort saves than will saves, and also so heavily benefit from defensive buffs.
I think that if dispel magic did not incur impairment, it wouldn't be out of balance, but that as it stands now, it is a clear obvious choice concerning every single end-game PvP. If anyone would rather choose slow/weaken/poison/blindness over this
Quote:
A repugnant male sprite utters the words, 'eugszr waouq'.
You feel less righteous.
Your rage ebbs.
You feel weaker.
You feel yourself slow down.
You feel heavier.
You feel less righteous.
You feel less protected.
Your force shield shimmers then fades away.
Your spirit companions leave you.
They're a crazy person.
tl;dr Dispel magic shouldn't have impairment because it's already powerful enough.