"Grants 5 gold for melee champions and 3 gold for ranged champions for each basic attack against a champion" (5 second cooldown)
The 5 second cooldown, alone, persuaded me to not pick up the mastery.
5 second cooldown when normal attack speeds for support vary from 0.6-1.0 (without zekes AS buff). That only offers a maximum of 60 gold for melees or 36 gold for ranged supports a minute. If perfectly executed, it would only be slightly better than a gp10 for a melee support and far worse than a gp10 for ranged support.
So what are supports supposed to do? Attack a enemy hero every 5 seconds? Hit once and run away? Because once you start the cooldown, there would be no point to risk dying to continue hitting an enemy champion.
And if you poke once, the enemy player can always just count out 5 seconds and expect another poke, so they can prepare to nuke the hell out of you when the cooldown is near 4 seconds.
Remove the cooldown and then pickpocket would be a worthy investment.