Well I'm pretty new here, so I'm not sure what the regular stuff is, but it seems there's a pretty heavy prejudice against 'BL'. I can see someone not putting it in a label/sig/etc for that reason. I suppose people might toss in that sort of label for fans, but frankly I'm not going to read something just because it has a specific kind of porn/relationship.
Of course, some people are looking for that specific kind of porn/relationship and yes, it can be helpful.
Now this is just my perspective and I'm not yet involved much in the site enough to know what goes on regularly, but I can say as your average guy who tends to prefer guys, if I walk into Barns & Noble and I'm looking for romance fiction, I'm not heading to the typical 'romance' section. I'm gonna head to the gay/les section. Hetero relations can sort of weird me out sometimes, so I would rather it be more in my field of comfort. On the same token, I usually don't read yaoi/shonen ai because I'm not a woman and it doesn't appeal to me. I don't much like feminine males in my stories. If I want something dirty then sure, I might go for some bara (to be blunt) or something more... for guys. But I'm gonna want a label -somewhere- that's telling me what I'll be getting into.
As for this BL stuff (which I assume is 'boy love') I'd probably think it was shota or something yaoi. If I see GAY MEN or HOMOSEXUAL or 'dude romance' even, I'd be more inclined.
Some people like to write romance. Some people like to write porn. Some like a more even dose of stuff with one or the other mixed in, and some don't like any of that. A label exists for a reason, either way. It gives you a perspective of what you're about to be getting into. If I pick up a DVD at the store and it says it's rating M or R or whatever due to graphic sexual scenes, I'll probably put it back, but hey, I'm glad to know it had the label to tell me. And someone else might just find that label dandy.
And maybe people write yaoi/BL/whatever purely to get fans, but so what? It's not hurting me any. I personally like more in-depth stories, but that's just me. Everyone has a preference, and every preference (just about) has a label to help us find it. People might abuse it, but that doesn't mean it isn't useful.
And that's just my long and drawn out thoughts on the subject.
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