It's usually just a complete waste of time regardless. If anyone ever catches a whiff of something I listen to, it's always "why do you listen to that screaming shit?" The real question is, why do you only pay attention to vocals when you listen to "music?" Meh.
I have one friend who listens to some similar music. He originally listened to bands like Slipknot a lot and I shared Opeth with him and since then I've opened him up to a lot of good music. He's a drummer, so a lot of metal impresses him, but he doesn't really understand atmosphere or the more emotional Agalloch type bands so much and doesn't seem to take music that seriously. He spends 0 dollars on new music and never searches for bands, etc. I've showed him everything he listens to, but his lack of dedication is a turn off and anymore I don't really care about helping him along.
That lack of dedication that he has though, is simply the problem with EVERYONE. Metal takes a lot of dedication to be a fan of, plain and simple. We're all willing to throw thousands of dollars into CDs, shirts, shows, etc. We don't mind listening to music that we know everyone else hates and we don't mind if people think we're weird, crazy people because of it. The music becomes a serious part of our life, not just a passing hobby, it's something serious and real. Most people are just too lazy or indifferent to make that sort of commitment.
I've shown a few bands to family (sister, cousins, mom) but it's mostly a fruitless venture. My cousins think some of it rules, but don't really understand it and let it just pass by. My sister went through a real awakening life period and during it, I helped her along and she got into some of my music, but I don't think it stuck. Sometimes it's just fun to find a fairly tame band or try to match one up with a person's taste to see how it goes, but at this point I have absolutely no one that shares my tastes. Hopefully that'll change sometime, I've just got to find the right girl.