While it's not a requirement to play the game casually with your friends, if you have valuable cards, or if you're playing in a tournament, you absolutely need to have them
The Spirit dies as a state-based action due to having 0 toughness
As if that wasnt enough, Cemetery Prowler also reduces the cost of cards of the same type as it has removed for you, allowing you to cast your own spells for one generic mana less
Avoid Mind Stealer since he can play with a copy of your own deck (although it's fun to have him cast Contract and then win his anted copy of your own card!) Paladin is also touchy, with Karma and Northern Paladin to work against your black deck even before he starts with his Magical Hack and Sleight shenanigans
And thats just because mutate is such a fun space to play in, with multiple commanders that can take advantage of this spider
A bittersweet dirge is sung