That's only because you use different types of media then. Cause removing 1 big filter sponge is even easyer than picking up trays one at a time, not to mention the fact that you would be able to pack ALOT more sponge into the canister without using the trays.
Hmm maybe this could work in my FX5, since the motor is at the very bottom of the canister
No, it doesnt have anything to do with media selection. The trays serve a flow function as well as a containment one. On the xp's, both the input and output are side by side on the top. The trays make sure the water flows all the way to the bottom of the canister before returning back up through the media... inside the trays. And there is very little wasted space in the canister with the trays in. You could not pack much more sponge in one of mine without the trays, aside from the fact that the media I'm using is much more effective than sponges. Not knocking sponges mind you, just pointing out the fact that there are more effective medias available.........
It would be worth a try on an Fx5 though, seeing as the design is different. I wouldnt fill it with sponges though. My choice would be pond matrix or ceramic rings. But then Id need some kind of bags or...........trays