Generally, I label things as slash or saffic just because I don't want to get the "eww ur gross" type of responses. That way, if somebody thinks homosexuality is an abberation or whatever (though my experiences in fandom have shown that 95% of people are pretty open-minded), they have an ideaof what's involved and they can either steer clear or get a "you read the warning, now piss off".

I think the main reason for labelling things for anything is that it lets sensitive readers know if there's "questionable" material inside -- and to some people, slash and saffic are definitely questionable, or they just don't enjoy reading it. That doesn't really bother me, mostly, because as long as they stick to their narrow-minded little worlds I'm happy to be completely ghey in mine.

(And you know, I was just thinking last night of all the weird double-standards there are for het and slash -- namely, how people get rabid and weird if you say you don't like reading saffic or slash and accuse you of being a homophobe, whereas if you say you dislike het, there are legions of people just waiting to leap in and agree with you. And, like you said, the way that slash needs to be labelled but het doesn't. Little things like that irritate me to no end, because everything in fandom is down to personal taste so all bases should ideally be covered.)