The use of Nakama in the subs doesn't bother me at all, the same goes for other words which warrant being used for translation. I don't see why people complain about it when they're actually watching something spoken in Japanese- it seems quite hypocritical to me. I find it a little cold when no Japanese terms are used at all.
However I do find it infuriatingly annoying when people start treating these things as a law and refuse to understand that it can't be applied to everything. The idea of using untranslated words like Nakama for dubs or translations is pretty ridiculous (names are a different matter, I think). Translating them is fine, I just find it annoying when it's not translated proparly or when there's only one substitute for a word. It doesn't just have to be 'crewmate' or 'friend' or whatever, just pick what best suits the situation. I don't see why people get so wound up about deciding on what should be the definitive translation.
As for the cultural argument, I completely agree with it… in other situations. I don't think it really applies to One Piece or anime. You can't really deny the fact there are millions of kids who love to say things like 'baka' simply because they think it makes them cute or quirky or whatever they imagine it to be, and it's only really these people who use such terms in real life. It's just a sort of fashion, I doubt it'll stay around long enough to become part of our culture.