More cards mean less chance to draw any separate card with summoning. Look at Dragons, for example - you never know what dragon will be summoned after Dragon egg, i.e. you're not sure if you'll be able to utilize it. When you have 6 uncommon and 1 rare Brigand - cards like Pirate bay are strong as hell, cause the chances to get Master thief, Prince of thieves, Hungry dead or Corsairs (and then Master thief etc.) are very high.