He is not weak at all
He has absolutely imba scaling spells compared to any other caster.
Soul catcher is absolutely devastating spell and actually much easier to aim than most people think.
Disruption is a terrific all round spell
Demonic purge offers slow,purge,damage all rolled in one
Poison is a terrifying lane spell.
He is the single most powerful laner in DOTA.I never lose a lane with him even against OD/Rexxar/Lich/Bat, and wreck havoc to agl hereos in mid game..
Disruption gives your team insane DPS.
Anyway, thats my feeling. Changing Purge damage to DPS is ok..But increasing its damage is unfair