It's a marketing myth spread by the filter manufacturers. My Penguin Mini (100) filter cost $18. If I replaced the cartridge every month as they suggest, I would be spending an additional $22 a year - more than the price of a new filter.
As Rich said, just rinse and reuse.
I do this every two weeks with my Penguin cartridges - they eventually fall apart in about three to four months. I don't concern myself with the carbon in the cartridge since I'm not medicating my tanks. The Penguins have biowheels, so when I replace the cartridge, I don't lose all my bacteria.
AquaClear filters are a bit more versatile since you can use sponge or filter floss - both can be rinsed and/or replaced in small quantities every month or whatever.