They're probably the most useful light modifier. You can shoot through them, of course, but you can also bounce off them (with some light loss). I've even made a paeudo soft box by placing a reflecting umbrella on the other side (using tape on the center shaft to hold them together), and placing the flash in between the two facing the reflective umbrella. The light bounces off the reflective umbrella and then through the shoot through. By using a slightly larger shoot through, you can eliminate most of the leakage.
When used as a normal shoot through, it acts like a middle ground between a softbox and a reflective umbrella. It's not as soft or even as a softbox, but not as hard as a reflective umbrella.
If you already have a lightbox and reflective umbrella, they're probably not necessary. But if you don't have either, they're a cheap way to get two in one.