People are generally fine with Expropriate from a power level perspective, even if many find it boring, but I think a plurality of decks that are power level 7 and up eschew big mana stuff because they don't need it, if you can win with Rite of the Raging Storm then you don't need Expropriate
Technically, both cards can do the same theft effect with a small difference: The creature half can exile a card from an opponent's hand, and they pay its mana value to have Valki transform into it
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Those are the main cards the Kavu is a counter for but there is a lot of incidental life gain in the cube which makes this a pain to play against
Card advantage one turn, then pumps your creatures the next two turns
50 card dividers help you sort your cards more effectively, and the upright orientation makes it easy to flip through them and quickly find the ones you need