It's not the CDR that made it viable it's the building up 3 stacks of passive per 30s instead of 1 per 10s+ the mana regen (the pros are all selling it lategame in favor of talisman, for the active and higher CDR). Combine that with things like morgana W and you can actually use the passive effectively.
Either way the buff is huge and means poke supports can go far more AP carry-ish.
Of course the goldgen change helps. Which is why I said before Talisman had 'The best stats, active and gold generation'. Despite the fact you had to do nothing for said gold generation.
I'm just pointing out it was silly that the item lacked CDR when it was supposed to be the 'caster' option. It was also silly that Talisman gave MORE mana regeneration. [Now it gives the same when maxed, but Talisman does need an extra fairy charm so it's fair. Coin and Amulet gave more than Edge and Frostfang.]
I don't sell it for Talisman later in the game for everyone. For example when I'm playing Vel'Koz support I'm generally not in a team that
wants to rush in. Generally it's a poke team or something. So I'll keep the Frost Queen to enhance my poke slightly, while also giving an extra peeling tool, and also giving me rather high gold generation still so I can transition into more durabilty+damage. Sometimes you don't want to speed the entire team up, sometimes you just want to slow down that one guy who's diving your carry, and for SINGLE TARGET situations, Frostfang's slow is better than Talisman's haste is.
Of course, Frostfang can't compare when you're speeding up the entire team at once to chase or something, but I'm fine with that. Especially since a good Vel'Koz Q can slow 3 people anyway. He's ALREADY good at enableing the team to chase.
Also Frostfang is nice when you're a pushing support like Lulu, because when fully upgraded the goldgen dosen't get stopped by lasthitting, so you can happily spam Glitterlances in a seige and shove/countershove. But on Lulu I see it as more situational.
On any other support I would certainly replace it for Talisman lategame.
Easily the best champions for Spelltheif are Lulu and Vel'Koz IMO. I don't personally like Morgana support, because whenever I play it, I have to stay far enough back that my binds are easily dodged, and then I'm useless. [And as you might have seen yesterday, Morgana does not shut down Leona. Not one bit. Leona just swaps targets and kills whoever you DIDN'T shield and you're left going: 'welp'.] It's not a comfort pick for me in any way, shape or form. I prefer harassing supports.
