Both these cards are old and won't be reprinted and while they're not needed in order to have a successful Shrine deck, having them will boost the power of the deck substantially
Multicolor Abrupt Decay Again, strong permanent control, and can't be countered
So if you have ways to sacrifice creatures for value something that Grixis (blue/black/red) has plenty of ways to do you can get even more value out of them
This is a great value card that works well in black or Dimir steal decks, but I think is better as an on-curve four-drop than as your actual commander
5 Roil Elemental An excellent blue creature that can put in a lot of work in the late stages of a game, Roil Elemental is a 3/2 flying elemental for six mana
Cultivate is one of the best though, because it not only puts a land card into play, but also puts one in your hand