Of course I know Swain cannot tank not matter what, since he is a support caster with CC abilities. However this build can be used in Team Fights where you already have a primary tank. Swain being tanky himself will divert people's attentions to other champions, so that people wont want to target him right away.

Swain's usefulness depends on how long he stays in battle. The longer he is alive the more snares and slows and tormets he can cast, and the longer his ulti stays on, which indirectly, even though no matter how little of a damage it deals, does contribute quite a bit in team fights.

People that go full AP build tends to use swain for his Torment Laser Bird combo for fast killing, however because they die so fast, where the point of having a slow and snare is almost none existent.

I personally tried this build for 2 to 3 games before and it works. I last through the whole team fight and help our DPS get Pentakills. Sure I phrased it wrong by saying its for the purpose of tanking, however the main point I had been getting at had always been survivability.

I do not think that this would work. Yes you can have you ulti on for a long time. But even with a pure ap build, your ultimate does not make you tank, even if you have 500 ap. You only have three tanking items, and since Swain is a kinda wierd tank, I do not think that this build will be able to tank in a teamfight. I am not going to rate the build, since I am not going to try it out.