I've never felt like I needed a skirt-guard and I've ridden in many different skirts and dresses. I always wear little shorts under them (they are sold many places, but American Apparel has plain ones which I really like), so usually the skirt gathers a bit under my butt and I don't need to worry about showing anything but leg to the world.
I'm not someone that wears long skirts (past the knee) so I can't vouch for those types of skirts not getting caught. Maybe on a bike with a slopped top-tube you would need one? I don't know.
Semi-related skirt issue: a relatively stiff or tight mini-skirt or mid-thigh skirt can sometimes get caught on the back of your seat when you dismount the bike (if you weren't sitting on your skirt while riding). It's less an issue of showing people your butt (well, so long as you're wearing shorts underneath) and more an issue of getting stuck to the seat and being thrown off-balance. Has this ever happened to anyone? It's kind of a funny problem and happens to me in the summer sometimes. Luckily I have never fallen!