However, if you level-up the Active Pokemon and it gets switched (or it retreats) later, the level-up card is still retained. You just can't level-up directly from the bench (without the use of Level Max or Promogon-Z).
I guess DNA sort of beat me to it but yes there exists a Porygon-Z promo that has an attack that can level up 1 of your Benched Pokémon, so you can use that, too.