Primarily, newer sets are more available and generally less expensive than older sets
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Just being stable won't allow you to deal with every deck If you aren't stable, you might lose to opponents you could have beaten If you add uniqueness, you might catch opponents you would lose to off guard and win If you add too much uniqueness, your stability will drop significantly What a contradiction
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However, Pokemon TCG is so draw-oriented and fast-paced that potentially whiffing on a single card completely, with the chance of a very strong positive outcome, is actually worth it
The series debuted in 1993 and laid the blueprint for almost all other TCGs that followed, such like the hugely popular Pokmon TCG and the recently released Disney Lorcana