There is not a single reason to upgrade Spellthief into Frostfang right now
Since the gold generation nerfs (2 from 4 every 10 seconds) there is no reason to upgrade Frostfang: you will be spending 500 gold to get pitiful mana regen, 5 ability power (wich basically means nothing), and +5 damage (mitigated by armor and/or mres, so it's basically nothing) on every proc.
In most of LCS, LCK and LPL matches, you will see the support starting Spellthief, then either selling it midgame to buy Coin (wich has got a way better active), or simply sitting on the T1 Spellthief whole game long, simply because upgrading it to T3 while sitting on T2 for 10+ minutes isn't worth it.
This tremendously hurts AP support or poke-oriented supports like Sona or Karma, because it kills their gold income while not providing a good T3 option, while a tanky support can just build Relic then upgrade it to FotM while having a good gold income with Targon's.
In most of the games the income given by the T2 Frostfang is just not worth because you will be roaming as a support most of the times, so you will be rarely able to abuse the buffed procs, while you will greatly benefit from upgrading Coin into Medallion simply because Medallion gives +2 gold every 10 seconds compared to Coin (wich gives 0), so there is actually a reason to upgrade it to T2, while there is no reason to do that with Spellthief.
Poke supports are even in a bad spot right now (the only one you will see is Morgana and she's not even that pokey).
My suggestion is simply to bring the gold generation back to 4, or 3 at least.
at least a bit.
in lane, and I think that champs that aren't based around frequent poke such as
can do well with it.