C band frequencies are much less susceptible to rain fade than are Ku bands. (Not immune, just much less susceptible....) Generally speaking, attenuation due to rain increases as the signal frequency increases.
Your comments about the beam being "wider" are incorrect. But the wavelength does increase as frequency decreases, and there is a relationship between wavelength and raindrop size that contributes to rain attenuation....
I do not think Dish worries too much about those in "fringe areas", as they can not provide service to Canada, Mexico or the Caribbean (except Puerto Rico).
[Should also mention that, for the same size antenna, Ku band antennas will have higher gain than C band antennas.]