I've been playing since 1.8 and I've never seen either a zombie or skeleton wearing any diamond armour nor has any ever appeared in my mob grinders.
Although it sounds good in principle to have an infinite source of diamond armour, it's rather pointless now that we have Mending. I've been wearing the same armour for ages. I've only ever used two sets since Mending became available - I fell into some lava once and didn't get out quickly enough.
I never liked mining so Mending has been a huge boon.