There's a difference between getting good at skills you're going to use, and getting good at skills because "you're supposed to to be a 'real' musician" or something. If you want to improve at playing a particular part, and you're only ever going to play it under tons of distortion, time you spend playing it clean isn't going to help you play it in the way you actually want to play it as much as playing it in the way you actually want to play it.