I think its a good decision too. Its all about what will last longer. What happens with carbon is that it tends to crumble and little pieces get carried off, often winding up in the tank gravel and getting gravel-siphoned out of the tank. The bacterial colonies are even smaller than these little barely visible particles (bacterial colonies are not really visible except as a brown stain on lighter colored media inside filters.) In contrast to this, sponge lasts much longer and offers a similar high surface area. Ceramics last even longer than sponges and can offer a very high surface area when in the form of ceramic gravel beds. (Ceramic gravel has a higher surface area than substrate gravel, although that too can be used in filter trays.)
Carbon is a great thing to have in the storage cabinet. If you think about it, gravel, sponges and such can do nothing if your water is yellow from bogwood tannins or if medications have been used. These are what carbon is for. Its a "chemical" media and can "adsorb" (collect molecules via molecular charge attraction) tannins, medications and the odd organic odor of unknown source. It is sometimes used in conjunction with fresh extra-fine floss mechanical media prior to a tank photo session, to help final water clarity.
~~waterdrop~~