As far as I'm concerned Wall of Denial is ALara BLock, therefore Standard legal, but correct me if I'm mistaken. If you DO put in the walls, mass removal is less necessary, but spot removal needs to be increased. Also, Cancel is just plain slow, crams up too much of your mana, spot removal can play its role in the end game for less mana (and also plays Clone role, as well). Soul Warden is just slow, also, unless YOU have an engine to pop out critters, because:
1 - Combo won't care about your life gain, if the combo gets going, it'll win anyway (alternatively, if they DO care about your life gain, they always have some form of spot removal to get rid of trouble creatures until they go off, so they get rid of them);
2 - Control plays too few creatures for it to matter (they're just too big), and if you want to say "But at least I can attack them a few times", then I'll say "Play Elite Vanguard, then".

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3 - Aggro decks. THESE are the ones that DO care about Soul Warden, they play a lot of creatures, and they run out of gas fast if they can't kill you. However, they all play some number of removal spells AND burn, and they WILL aim it at SOul Warden, so you'll rarely benefit from them.