My suggestion is that you add life stolen per hit to the resist helms, as this will actually provide merc utility/survivability of these helmets in various specialized situation like travincal (temper), andy runs (cure) etc.
Obtaining life leech on the armour slot for merc is extremely difficult (Boneflesh, COH and blood craft are some of the only options), and seeing as 90 % early mercs use insight, the helmet slot is the only viable option for life leech, which would actually make the helmet rws useful and the merc able to survive.