depends. nutshell essay here. the fatter you are, the less it's needed. But once you're approaching having your viseral fat level to be in the low %, it becomes required or your body fat levels will never drop unless you're starving yourself into the ground. And once you get to 0% viseral fat, if you want to go further to get that "toned" look, resistance training becomes required.. So this is a loaded question on how you define "sucess". Me personally, I like having muscles show through a low body fat %, so exercise and specifically resistance training is 100% required for success.