Spellthief's Edge isn't healthy or necessary in League of Legends
(All stats shared are from global, plat+ ranked data on lolalytics)
Let me lay out some data for you. In examining the problem with Spellthief's Edge, I think Enchanters are an illustrative example. In theory, Ancient Coin is a fine item for these champions. In practice, it does not work nearly as well as Spellthief's Edge. In the top 10 champs, sorted by tier, there are 4 enchanters (Soraka, Janna, Nami, and Sona). This is a big upswing from the last patch due to buffs for enchanter items (Forbidden Idol and its upgrades). But there is little tension between building Coin or Spellthief's on those champions. All of them perform better on Spellthief's than Coin, with an average win rate improvement of 3.86% (standard deviation is 0.77). Nami gets a whopping 5.6% win rate boost just from picking Spellthief's over Coin.
Of course, this problem is bigger than just enchanters. 8/10 of the top champions, sorted by tier, build Spellthief's as their gold gen item. Whether you sort by win rate, Pick/Ban influence, or pick rate, the majority of the top ten champions in each of those categories are Spellthief's users. Champions like Zyra and Brand, at first, were kind of refreshing to see bot lane because they were new and different. Now, many old players recognize this poke-centered bot lane meta as a key contributor to the feeling of excessive damage present in League.
Hopefully, at this point, I've convinced you that there's a problem with the item. Otherwise, Ancient Coin might be useful on champions other than Thresh and Blitzcrank (Blitz, notably, is equally effective on Relic Shield and Ancient Coin). While the numbers do a great job of establishing the issue, what I want to talk about is why it's a problem. Let's talk about Spellthief's mechanics.
On true mages, Spellthief's provides a "win more" mechanic in a way that the other two gold items simply don't. If you're a mage and you poke well, you get gold and you push the enemy out of lane. If you can't poke successfully, you're out lane pressure and gold, making it tough to come back. You don't have a lot of utility to do anything from behind on Zyra and Brand.
As bad as I think that is, though, I find Spellthief's on enchanters even more damning. When Soraka builds Spellthief's, the most successful way to play is to aggressively poke an enemy down try to secure kills. That's a problematic playstyle that clashes badly with Soraka's champion identity, imo. Enchanters have been pushed in a direction that makes them more interactive in the last several years. Soraka needs to poke to maximize her heals. Janna got W buffs and a passive change that encourages trading. I like the interactivity, but I don't like the damage that Soraka can do now. It dilutes the experience of playing an enchanter to put that much power into their poke. There are other problems under the hood there, perhaps, but Spellthief's is a definite contributor.
I understand the design principle here. Spellthief's forces interaction. But, in practice, it leads to a stressful laning experience rather than a few tense moments brought about by that interaction. The team with more kill pressure wins, because there are so few defensive options. That's why everyone complains about League's damage.
I actually got the idea to write about this because of Nexus Blitz. Someone at Riot must understand this, because there is no Spellthief's alternative on Nexus Blitz. Targon's and Coin have their alternatives in Nexus Blitz, and I have had a blast playing support there. When the opponent does have a mage support in lane, they have to sacrifice either their gold gen or some AP. That's fair for how much pressure they exert. I had a fun Vel'Koz game where I did a non-oppressive amount of damage after building Philosopher's Medallion for laning. That game, I had this stunning realization that AP supports can be fun when they aren't blowing you up with two spell rotations. I was creating space for my marksman and applying cc for my team, rather than racking up kills and stacking my mejai's, at least during laning. Later I had damage and did exactly what you would expect Vel'Koz to do, but I wasn't an oppressive laner, and that's a big deal to me. There's a reason that League of Legends: Season 2 didn't have a gold gen item that gave AP.
The easy conclusion is to delete Spellthief's. I am sure there are alternative answers to the problem, but I see little redeeming value to Spellthief's in its current form.
Got another perspective? Think a small tuning adjustment would fix it entirely? Please comment! I would love to see other's thoughts on the issue.

like yeah it no fun at all to fight against cause it constant harass from the support. but the reason why it just overall does so well is the fact that it doesn't need much effort. If you compare the coin
and spellthief
which makes more gold? suprisingly spellthief and that just cause the fact that with the coin it a chance of gettin Gold or mana token as for spell thief you get 10 gold per spell cast meaning if your sona who has a large range for her Q your always getting 10 gold ever 10 seconds or less then upgrade both them
you got one that has a chance of dropping a gold token worth 45 or getting 20-60 gold per spell cast ever 30 seconds. the only difference between the coin and spellthief is the fact that the coin you can kill minion and earn tribute as for spellthief that a not until you complete the quest
+ 300g)
+ 250g)
Remnant of the Ascended (rework)
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