I personally think the nerf to swain came out of theoretical math that was taken vastly overboard. I can't even remember the last time I saw a Swain, let alone one do well aside from ARAMs, and even that's rare, most people roll off of him.
In theory, Swain can maintain almost constant uptime on Deathfire Touch.
Torment deals damage over 4 seconds and amplifies damage dealt over it's duration, so that's 8 seconds of uptime if you count it being continuously refreshed and then active for 4 second from the last instance of damage.
He can then rotate in Decrepify for an additional 7 seconds of uptime on Deathfire Touch. 3 seconds of continuous refresh from the spells DoT, then 4 seconds after the last instance of damage. By the time that's over you're able to throw Torment at them over again. So there's the theoretical cycle of endless uptime.
Furthermore I personally believe that if you're not CCed hard into the snare and you manage to get yourself snared, you deserve to get obliterated, there's nearly a full second delay before the ability triggers and the ability has an 18 second cooldown at level one on a champion that basically doesn't build CDR in most cases. At it's current state it's just a waste of mana, because even if you land it, you're not going to do anything with it afterwards as they're on their merry way to safety before you can get a decent rotation.
Edit: Returning to the theoretical math, it doesn't make sense to try and maintain a 100% uptime on DFT as if you don't combo your abilities with Swain you deal embarrassingly low damage.
