Makes sense for Aang to airbend far more than the others. Who's fought an airbender in the last hundred years? Plus, it's his native element that he's already mastered. He has far less experience with the others.
It's pretty interesting, given the above, that Korra is much more balanced in her use of bending. She's been practicing the first three forms since she was a child and mastered airbending during the course of the show. The writers seem to have put some thought into how both characters would fight given their respective backgrounds.
The interesting thing is how much Korra uses airbending in S2 after having mastered it. And how in the last two seasons, her native element is the one she uses the least.
I think you have to consider personality as well. Aang is very much like "air". He's free and spiritual. Korra, on the other hand, is nothing like water. She's impulsive and hotheaded. It only makes sense that her most used element would be fire.
I mean, respectfully there should be very few airbenders in the world post Aang as well. So it should still work well as very few would have ever sparred with them.
It's statistics, a form of math the looks into the science of averages and how you can extrapolate information from the data. Korra just has what is called a larger sample group.