I initially thought the same thing, but after the 1st time I used an iron decon product on my paint and decided not to agitate it before rinsing it off I felt like I didn't get the full potential out of it as soon as the water hit the panel. I guess it may have been how it seemed to take a bit more than usual to rinse away or something to that effect.. But I immediately thought I should've went over it at least gently with a mitt to get the full potential out of it.
The 2nd time I used it [this past weekend] I did gently go over the paint with a mitt and it felt like the better way.. Similar to what it'd be like if you chose to agitate the wheels or simply rinse the cleaner off, but different. You'll see..
And yea why even consider using a sponge? Use a mitt, you don't have to go full strength on the scrubbing, just enough to work the lather around, then rinse it clean.