the buttermilk brine method I use is based on a recipe from Gary Wiviotts book Low and Slow, plus a few discussions with the man himself. The buttermilk brine is washed off before applying the rub in this recipe. Otherwise the chicken can be salty to some. I guess less salt could be used and the brine not washed off allowing the brine and rub to form a nice crust when grilling vs smoking .
For deep frying soaking chicken in buttermilk, or even a non buttermilk brine is pretty standard for me. I am doing some chicken wings to day for fried chicken and waffles, and the wings will be soaked in buttermilk, and hot sauce for a few hours before hitting the deep fryer.