In looking through the forums I've noticed that people are frequently shooting for a player deck size of 50 cards. On page 9 of the rule book it states, "For advanced play, the cards in these starter decks can be combined with one another, or further developed with cards from Adventure Pack expansions, to create 50-card tournament decks." This implies that a proper tournament deck should be composed of 50 cards. However, page 27 states, "A tournament deck must contain a minimum of 50 cards." So it sounds like you can have every single card you want in your deck with the 3-of-a-kind being the only restriction.
So I wonder, do people compose player decks of 50 cards because they think (incorrectly) that 50 is the limit? Or is it because limiting your deck size increases the chances of you drawing a card that is key to your current deck's strategy?