Well, actually, they are.
1) They let you "adjust" your life pool for infernal contract (like "cabal pit")
2) Reaching your "threshold" could be a matter of life or death (for cabal pit)
3) Considering that you could be "on par" with other attrition based deck, it is also important to "draw" a bit less "lands" when using infernal contract.
The difference could be minor but even if it nets you just another active card instead of another swamp, it is huge. Nothing is worse than dying because your infernal contract give you too many lands instead of active cards.
Your life pool must be used as a "ressource" once you get use to it.
You don't have to wait to play "infernal contract". Sometimes, I go for it as soon as I can. It is extremely dependent of the matchup you are facing, though.
The list I shared is, by no means, a finished one. I'm pretty sure MD can still be adjusted (a 5th draw spell and another wincon for example are a few things I would want to include). Sideboard could certainly be improved by a very large margin.

