I doubt that anyone claiming full asexuality is doing it to be trendy, just the in between labels that would give someone an opportunity to have their cake and eat it too, which you already said yourself gets picked up by people who probably aren't serious. I'm sure there are people calling themselves demi or whatever that do fit actually fit the description also, but as you said no one who isn't family or wanting a relationship with that person is likely to care at all, so if someone is going around waving whatever variation of pride flag and making a personality out of it I'm probably not going to take them seriously.

Yes, they're harmless enough that I could just let them feel special anyway and go about my day without comment, but if I was that kind of non-asshole I wouldn't hang out in Tumblr in Action lol.