I believe he's refering to the pre-filter for the bio-balls of a wet dry? If so, thats only designed to trap any large debris that would build up on the bio-balls creating a ton of detrious build up. So it should be perfectly fine to just throw out. Your major bacteria colony would be on the bio-balls.
On a power filter with multiple filter pads, like beaselbob mentioned. You'd only want to replace one half at a time. Example; on a skilter 400. Theres two pads, replace on of the filter cartridges... wait a few days-week, than replace the other. By staggering, you're allowing the bacteria to colonize the other also, before pitching the old one.