What do you think are the requirements to be a Gym Leader?
I don't think its just about being powerful, there's something more profound behind that strength. I guess it boils down to friendship and harmony with others, people and pokemon alike, which sounds really sappy but has been the core of the entire pokemon series since its beginning.
There has always been that rival who's a big douche, which personifies the wrong way to gain strength and then there's Ash, whose not all that bright or good but perseveres with the help of his friends. In the anime its not even necessary to always win the gym leaders to get the badge, when Ash proved his connection with pokemon by risking his own life to save a pokemon.
I think gym leaders more as a test of will. A particularly fitting quote for this would be: "You do not truly know someone until you fight them." A gym leader doesn't need to use his/her full power to test a trainer, as long as he/she can see whether the trainer has potential to become a great trainer one day. I mean how does it make any sense to have gym leaders with only a few lvl 10-12 pokemon? It doesn't, they're going easy on you.