I am personally trying to learn how to play as many characters as I can stand, and learn their AT's and strategies. My theory is that if I know how to use all the characters on an good/advanced level, I will know how to counter all of them and also will be familiar with their AT's. Sure you can learn about characters by playing against them, but you certainly learn a lot more if you play as them. I know every Ike approach, all his spikes, AT's, SAF's, approaches, recovery, as well as all the hitboxes and knockback for all his attacks. Any Ike main will have struggles going against me, because I know Ike inside-out even though he isn't my main and most likely they won't know my DDD as thoroughly. I want to be able to have this advantage as much as possible, so I am working on learning all the high tier characters, as well as my personal faves who are low-mid tier(ganon, ness etc.). Currently I am trying to figure out why many people say Luigi is amazing by learning how to use him. Off to the Luigi boards for me.
It's okay to play as many characters, just make sure your learning some character specific AT's. If your using Snake without grenade dropping, Toon Link without SHDA's, Lucas without B-sticking, DDD without chain grabs, Ike without Grand Vipers, or Bowser without infinite jumping, you aren't really "using" them, and they should be played with less unless you plan to learn how to do those things, or don't care about getting better at the game.