I can only second the things mentioned and perhaps go a bit specific in terms of stifle:
Stifle is the absolute go-to card imo:
1. Against Midrange unlike some Delver decks that play only Daze/Wasteland you don't get screwed because your opponent has like 3 basics he can just fetch.
2. Against Storm Stifle is almost a Flusterstorm thanks to the ability to stifle the storm trigger.
3. Against Show&Tell you prevent them from fetching for basics, making your Dazes not dead! Also Stifling Griselbrand or the Annihilator Trigger can come in handy. Postboard your Therapies are super easy becasue you just name Show&Tell most of the time.
4. Against Miracles thanks to Gitaxian Probe you at best see what they're dealing with and can decide wether to play mana denial or more midrangy, denying their Terminus.
5. Against Lands/4c-loam preventing the opponent from wasting you out of the game for just 1 turn longer can often win you the game, especially with a YP out. Also in the longer fights where the opponent has Wastelands, Stages and Ports and you have Wastelands it can prevent him from getting an edge by preventing him to copy DD in response to your wasteland e.g.
6. Against other Delver decks thanks to both sides playing with Daze (often MVP there) Stifle really allows you to control the mana of the game, often times acting offensively or defensively, depending on your hand (do you need the mana he's trying to waste? do you want to cut him off red so you can stick a YP? Did you already waste him?) 5 and 6 imo often times getting neglected, forgetting that we actually play a pretty fragile manabase ourselves, especially the 2 underground seas can get cut off when the opponent has a good draw.
Against Death&Taxes, MUD and etc. Stifle is just not really good, even though it has some narrow implications. But that's fine, that's what we have a sideboard for.
To the Cabal Therapy in the SB and why:
- Often times you want to hit your CB in the early turns. That's not really possible against an unknown opponent who perhaps just plays a fetchland and passes. Stifle on the other side allows you to play to the board, stifling his land in the early turns is just a "good" play, no matter what he's playing.
-Personally I find myself bringing in CB a lot. But that doesn't mean it should be in the mainboard. It's just that postboard I have an exact plan what I want to name, often times naming cards in the blind that are strong against me, but also possibly 4 ofs, e.g. Hymn to Tourach. Preboard all you can really name is brainstorm because that's what blue decks play, but there's nothing worse than naming Hymn against a player that played an underground sea only to find out he's playing Stifle-BUG, Grixis or Storm. You just wasted a card and it's a long time until you get a Pyromancer token sometimes.
lack of artifact destruction in the SB:
- Personally I wouldn't go without, but that always depends on your local metagame. When you look at online lists I guess that there's just not a lot of people playing MUD or 4c-loam. Stoneblade players on the other side just get Cabal Therapied, so Artifact destruction is really only necessary against Chalice of the Void.
Just look at this:
http://thegraymerchants.com/?p=1400
The first deck you actually need artifact destruction because of Chalice is 4c-loam on 12th place and they don't even have Chalice on the first turn most of the time, giving you time to dig for a FoW. Postboard you can just name Chalice on your first turn and propably be fine.
On the other side there's a lot of Delver, Miracles, Elves and other stuff. All in all Ancient Grudge is just not really viable.
I hope that above I could explain why we do play enough counterspells thanks to Stifle.
The lack of removalspells gets negated by the fact that most of the time it's just us that have the biggest/most creatures (as mentioned above): We play 14 creatures unlike most Delver decks that play 12.
Gurmag Angler is really hard to deal with and the only Mainboard Legacy cards to deal with him are either White or Liliana of the Veil which is horrible against YP (and can be stifled). Against BUG decks that rely on Disfigure and Abrupt Decay Gurmag Angler is the MVP.
Pyromancer on the other hand is a card there are not that many good 1-for-1 answers against in most mainboards once he gets 1-2 tokens, at best Terminus or some Spell Snares and Forked Bolts. The thing is that these answers have to come immediately, YP is very much like Dark Confidant here. He doesn't give you card advantage otherwise though, he just kills the opponent.
Removal to Goyf: The logic behind this is that with YP and Angler you just block the way. Submerge is just here to buy time or to return Tarmogoyf so you can swing in for basically lethal.
That said, I still like my 3 SB Abrupt Decays thus far, my metagame is quite diverse and having answers to creatures, artifacts AND enchantments is just a luxury I want to have. Again this is something I can only pull off thanks to stifle, otherwise my mana would get wrecked by other Delver decks, even with 2 tropicals and 2 undergrounds.