I agree with lalimacefolle. Scales are the spine of your music. I bet almost all great (metal/alternative) guitarists know the atleast the modes, a great amount of these guitarists know also the "not so known" scales like the Diminished and the Augmented scales.
I think that you should try to learn as many of these scales as you can (or as many as you want) because they give colour to your music, and that's the main importance.. Not that you can brag to your friends how many or how wicked scales you know, but to give your music some feeling and something that makes it worthwhile to listen.
But hey, that's just my opinion.