Having 3 toughness is a much bigger upside than it initially appears
Derevi leads my Commander Equinaut deck, and there she's a fun player that can get an Aura Shards trigger, make a flying blocker, swing for damage off Tolsimir Wolfblood, and such
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destroy them, and if you destroyed a face-up Fiend monster(s) by this effect, this card gains ATK equal to half the Fiend monster(s)'s original ATK, until the end of this turn
SR Super Rare UR Ultra Rare SE Secret Rare UL Ultimate Rare PS Platinum Secret Rare CR COLLECTOR'S RARE QCSE Quarter Century Secret Rare Xyz Encore X SPELL Quick-Play Target 1 face-up Xyz Monster your opponent controls that has material
So while Lucario isnt worth building a deck around, its absolutely worth slotting into any of your existing fighting-type decks