the MDF in the Rockler design provides a recess for the dust collector and (also) acts as a spacer for the sliding stop.
alot of folks face jigs & fixtures with sacrificial stuff like the MDF. were it wider it would also provide a clamp landing - clamping the aluminum can bend up slots/etc.
now, why the MDF with the slick white coating on a fence where you're trying to keep things still is a bit of a mystery . . .
I use a 2x6/2x8/2x10 clipped onto the metal table - it's long & wide enough I can clamp fences, stops, put in screws/bolts, whatever I need for the task.
and I just drill onto/into it . . . when it gets too ratty, I replace it - much more minor cost / trouble than the pretty stuff.