We all have that card that we just can't give up on, no matter how many times we've tried and failed. It calls to us, with its sweet siren's song…irresistible.
Smallpox is that card for me. At best, it completely decimates your opponent while you discard a Lingering Souls, trigger a Flagstones of Trokair, and sacrifice the Monk token that Monastery Mentor provided. At worst it does few to none of those things, but that upside is hard to ignore.
The primary issue with most Pox decks is that they are forced to devote so many resources to denying resources for the opponent that they don't have an effective clock, thus allowing the opponent time to draw out of their position. Dark Confidant and Monastery Mentor both stop this from happening, keeping the resources flowing in the form of cards or creatures that end the game in a hurry.
Planeswalkers are also there to keep the gas flowing in resource-tight games, with Sorin, Lord of Innistrad working particularly well with Monastery Mentor and Lingering Souls. Whether it's better than Sorin, Solemn Visitor is unclear, but I've always liked the first B/W version of Sorin, so I'm willing to go with it.
Second, Smallpox can struggle with decks like Affinity that spew their hands quickly onto the battlefield, leaving you little time to leverage your discard spells. That's what Mana Tithe is for. It gives you another piece of cheap interaction that your resource-denial plan keeps relevant into the mid-game but can stop you from being buried early on by the fastest decks in the format.
Honestly, Mana Tithe might be another siren card for me, because when it works, it always creates a story. It's never going to be as good of a story as countering a Cruel Ultimatum in the Top 8 of a Pro Tour, but trust me, any time you Force Spike someone, it feels great. Getting the last card in their hand because you've made them discard the rest is even better.
It may seem strange at first to see weaker black removal over Path to Exile, but giving your opponent a free land is about the biggest non-bo with Smallpox possible, so Victim of Night and Smother it is. I'm more than okay sacrificing there in order to play some of the sweetest cards in the format.