You're not too far off Sydian. Shaymin is really only used in Hydreigon Darkrai builds and that is because you can drop it out of nowhere and move the necessary energy onto it for a surprise KO. Shaymin does well to counter Terrakion as after you are down two prizes, Shaymin clean OHKOs it. Synthesis is never really used, it's all about the Revenge Blast. Shaymin also makes for a great final attacker as it can swing for 150 / 180 damage towards the final stages of a match. All in all, it is a niche card but it makes for a sweet tech in my opinion for the HD deck.