If I were a Paladin, a sword using role, I would naturally gain experience using a sword. So, it would follow that by playing as a paladin, the Sword Class skill would raise as well.

However, playing a Gladiator would not naturally increase your paladin skills. Because the Paladin and Gladiator are so similar, you don't need to spend a month using a sword and a few more skills to become a Paladin, you simply need to learn the abilities that differentiate a Paladin from a Gladiator.

So, obviously the dev team saw quests as a much more fun, interesting, and potentially more challenging method to introduce these abilities to players.

I agree with them and I think your idea is terrible.