I disagree that they are primarily designed for fry. They're excellent bio filters and do a much better job with physical waste than you might think. Some people swear by them and use them exclusively.
I love them because they're easy to DIY and as such they're very cheap (seriously - sponge, tube with holes punched into it, airline - that's it). They're effective, they're versatile and with no moving parts, they're much easier to maintain. I'm not sure it's possible to actually "break" one. They're perfect for low/no current tanks if you use an airstone rather than a powerhead - you can keep the lift tube high - just low enough to stay below the water line.
I'm a big fan of over-filtration, especially in a small tank. I keep an extra sponge filter seeded so if I need to move a fish right away, I don't have to worry about a cycle. Also, awesome for shrimp. When I had them, there were always 5 or 6 on the sponge, sometimes the thing was covered with them. And since I have a magnet in mine and keep it attached to the glass and raised off the sand, the bottom of the sponge was a perfect spot for the fry.
So yay sponge filters, IMO