This card is useful if you're running deck that purely focusing on fusion monster. I'm currently using KOG deck revolve around summoning Dark Paladin or buster dragon in the earliest turn as possible.
With balance skill, you are almost guaranteed to have your starting hand contains 1 spell card in 20 deck cards, if your 20 card consists of at least 5 spell cards.
(2) and with such condition, having only 3 polymerization is not enough to have consistency in those deck. To increase it, therefore other fusion supporting card is needed, like fusion sage, or fusion gate, hence making the spell card count to 5
It's not thinning if it cost 1 deck slot to search for 1 card lol
As an UR, definitely Swamp King would've been better as it plays 2 parts. Sage tho is still a necessity imo, but as a Rare or something. Seeing as Poly is a SR tho... I honestly think KONAMI don't think these things through too often tho.
For u who missed to get the second fusion gate for auto deck