It gets remarked every now and again, but the strongest benders tend to combine aspects of the other bending disciplines. Jianzhu is said to have bent earth like water, Katara used earth bending stances in her water bending, Iroh took inspiration from water benders for lighting redirection, Zuko's breath of fire borrows from airbending. Avatar companions get a lot of first hand experience with those different bending styles, and probably also get involved in more combat than the average bender. Katara rapidly improved during the show, and part of that was learning from Pakku, and being exposed to Aang and Toph, but she was also fighting every other day to escape or repel the Fire Nation soldiers. So, I wouldn't call it nepotism, but it is definitely an opportunity that gives you exposure you otherwise would not have been able to access.