Miriam @ Feministing takes up the question of "queer" as an identifier in a post from last week, What's the difference between lesbian and queer? and invites readers to share in comments what the word means for them and what words they use to speak about their identity.
From my perspective, there are two main reasons to use queer as an identifier. Queer is not as specific as words like lesbian or gay, and it does not explain exactly either your gender or the gender of your partner.
Lesbian implies pretty clearly that you are a woman who partners with other women. You might identify as genderqueer, trans or gender non-conforming, so that kind of specificity might not fit well. Or you might partner with people across the gender spectrum.
If someone partners with people across the gender spectrum, "bisexual" may not feel appropriate because it implies there are just two genders (bi meaning two). Additionally, if a person might not identify themselves with a binary gender (male or female) then a term like lesbian or gay might feel limiting.
Queer is an umbrella term, it really implies "not straight" more than it implies what exactly someone's sexuality might be. It's also a political term and many people use it as such, to imply a particular set of political beliefs alongside their orientation.
You can read the whole post at Feministing as well as the comment thread, which is where a lot of the conversation takes place.