girls like lunges because they have idiotic fetishes for colorful dumbbell weights and a crippling fear of barbells. squatting is a superior exercise in a lot of ways.
the squat itself has several variations - front squat, full squat, hack squat. that's what you do when you need to mix up your squat, but it activates so much of your core that it's very much an essential exercise.
*now that i got my bro post out of the way, i actually went and looked up lunges and the muscles they activate.
dumbbell lunge -
http://www.exrx.net/WeightExercises/...s/DBLunge.html
barbell lunge -
http://www.exrx.net/WeightExercises/...s/BBLunge.html
seems like you could feasibly replace a squat with it if you absolutely had to, but the nature of the movement means that it'd take twice as long. it's also a more treacherous movement if performed with a barbell (like god intended), and a little unwieldy to perform with dumbbells (i squat ~222, imagine holding a 100+ lb dumbbell in each hand while prancing about). it also looks like it'd put more stress on the knee, which would be the only reason to avoid a squat in the first place (though if you squat correctly you won't put a great deal of stress on your knees).