Since your deck will be full of instants and sorceries anyway, that prowess ability can grow it into a 4/5, 5/6, or something waaay bigger depending on how many cards you can play in a turn
There are some reasons why its vital to be careful when choosing the holder, as some binders can harm your deck
Bolas arrives, sorts things out, invokes a huge swing in the game and then sets about winning it for you
Some effects that instruct a player to flip a coin care only about whether the coin comes up heads or tails
1 Pikachu ex The Pack Art Doesnt Lie Pokemon wouldn't make their headliner Pikachu a bad card, which is why Pikachu ex has plenty of features that make it the best card to pull from this specific pack
A deck that uses Pitiless Plunderer only needs to sacrifice a token creature or use one as a chump blocker, and thats just the first synergy that comes to mind