Indeed, I used to think characters in the anime or even the games were just stupid. "Ash you only have like what.. an average of 4 or 5 Pokemon, period at any given time?"

But when you think about all that must go into taking care of an animal, unless they don't need to eat while in their balls (Which may or may not be the case), it becomes more realistic, you wouldn't want to be responsible for feeding twenty Clefairies because you just HAD to find one with the right Mark