I think more extreme bands should have clean guitars or acoustic guitars. I'm not saying bands need to go full out like Opeth, but 1 or a few of these on an album really give everything more dimension and diversity, and also a traditional feel. Acoustic guitars can be weird/dark/evil/menacing. The acoustic section on Dissection 'unhallowed' towards the end for example. Sometimes metal music seems to even get more evil with acoustics and clean parts. I like metal to sound like it's coming from nothingness/abyss. Sacrilege has some really sinister acoustics for example. Acoustics and clean guitars really do demenstrate someone's capability and diversity as a musician over a lot of techniques and approaches. If you can't sound good on a clean instrument that demenstrates you are not a good musician. Musicians showing what they sound like without distortion is very nice sometimes.