Pickpocket in it's current form is unusable on melee supports. Melee supports can't harass with auto attacks - if a melee support is auto attacking they are going for a kill. Why would nunu risk losing at least 1/2 his hp to run up to the enemy, auto attack once, then run away for 5 gold every 5 seconds. Ranged support can harass all day and get the free gold. A 2 gold difference doesn't represent the increase in difficulty for an melee to use this mastery.
Melee auto attacks on champions grant 5 bonus gold and ranged auto attacks on champions grant 3 bonus gold for each auto attack (5 second cool down)
Ranged auto attacks on champions grant 3 bonus gold for each auto attack (5 second cool down)
If a melee auto attack kills a minion both you and a nearby allied champion are granted the gold for the kill (45 second cool down)