1. You already should be relying on your adc to last hit for the coins, and if adc backs, then you cs and get gold anyways.
2. Having only mana coins could be beneficial to some casters when their adc backs
3. More gold for playing a support, when your with your adc (Which you should be for this item)
1. More gold for hitting abilities, this is beneficial for supports that should be seen, like sona and soraka, who should be backing with their adcs anyways.
This nerf could honestly be directed towards mid-lane mages being played bot lane and taking frostfang. Such like Brand, who wont be allowed to free poke and farm while the adc is away. IMO this isn't a nerf, but a buff to support players who are good at the role. This will definitely affect players who on off support, or don't play it at all and get autofilled.
And this might lead to Riot getting rid of the passive on Frostfang for taking CS, or maybe they will change it to where you only get the cooldown if the adc is within 1500 units and you take farm.