They are used, however there are problems with them. They are also completely overkill and not very useful for salt water.
1. They offer by far the highest amount of biological filtration.
2. If there's a power failure it could wipe out almost all of your BB.
3. It takes a lot of trial and error to get it just right
That being said, for our uses they really are pointless. Everyone worries about more biological filter media but don't think about it from an objective viewpoint. The tank can only support a certain amount of bacteria. Adding more media than the bacteria can colonize isn't going to make more bacteria grow. Especially when the entire tank can be biologically filtered from a single filter pad.
For salt water this is even more pointless since its really more focused on denitrifying bacteria rather than nitrifying due to our ultra low stocking levels and nitrogen control methods.
