These are the ones I use as well.
If you don't leave the strainer on and you push the prefilter all the way onto the intake tube, water will only be filtering through the bottom part of the sponge rather than being spread out, and it will quickly become clogged.
Oddly, the Fluval intake strainers are way too big for the Fluval prefilters. So I take the strainer off and slip the prefilter on about 3/4" inch. It would be better though if I found a narrower strainer that fits the tube.