The op has a point. It's ok to realize/accept your desires, but until you act on them you're still useless.
The touchstone for determining the graduation from uselessness is sex; surrogates like wet dreams or shlick-shlick usually don't count.
The useless lesbian is by definition the lesbian that is interpreting everything as a sign of friendship.
More than this is just plain lesbians.
IMHO that's not a comprehensive definition.
A lot of girls in yuri manga realize they are in love with each other but just don't know what to do about that. They confess and start dating and everything, but the idea of lesbian sex seems to be alien to them: they spend time together, go to the movies, eat parfaits, have study sessions... and that's about it. Sexual drive? What's that? Cunnilingus? He's that guy who led the Volscians to besiege Rome, right?
Even worse, some start lusting after their partners and get the urge to do sexy things to them, but don't say anything; instead, they do everything they can to repress their desires. Sometimes both of the girls in a couple are in this position at the same time, and their interactions become extremely awkward as they both keep thinking: "I must hide my impure thoughts! I must not touch her lest I lose control! She must never know!"
All of these are examples of uselessness, in the opinion of many fans.