Why do people build this on him? In theory, it would take a while for him to complete his quest since he only has one ranged poke tool, and to get any more stacks he needs to go all in, or attack the turret. I saw this in one of the professional EU games today, and I'm wondering if there is any specific reason why it's built on him.
Coin is better for solo queue.
I will only take spellthiefs on certain matchups against supports, mainly 
OR if I am being paired with 
. In these matchups you can easily kill them with the extra damage, or they are melee users that open themselves up for stacks quite often when using targon's . Against Ali and Leona you need Spellthiefs imo because you need to take advantage of their level 1 weakness.
Taking it is a huge risk though, because Rakan's poke is garbo and outclassed by basically any other poke support. Normally what I do is wait for the above supports to use their spells to use charges, then go in with W, auto and q, then leave. If you mess up or get behind early, you are not going to complete the quest for quite a while. Which is why in most cases I take coin.
Taking coin is never really a bad thing, but taking spellthiefs can be, so in solo q I just pick coin 90% of the time.
His AP scaling is 60/70/80/50%, respectively.
It's an aggressive item to run on him, alongside runes like Celerity and Manaflow Band. The counterpoint is that you'll have to build some kind of combat-sustaining items like Hourglass/Banshee's in order to dive fights and have a reasonable shot at making it out.
So with +10 AP at start, his shield does 40 + 8 + 0.8 dmg mitigation at level 1. If you take Revitalize, it's 49 + 2.4 for anyone above the threshold. Allies below I think 40% hp get 49 + 5.
