People use it because it's very effective.  I used it when I cleaned planes at the flight school I worked at in high school.  But using it on a GA plane is a different environment than on a helicopter at sea with a whole bunch of dissimilar metal parts. 
I get that there's a lot of lore out there about cleaning products, but I first learned that from my AM(H)C Det Chief when I was a Det MO, and he was pretty sharp on metal and corrosion.  I heard it again later on the GA side.  I personally don't use it on the belly of my plane, but obviously that's different than all the nooks and crannies of a helo.  I think not getting it thoroughly rinsed out in some places and exfoliation is where one of the big concerns is.
I thought it was outboard over inboard, which would technically mean you're correct...half the time.
Ironically, who did I learn that from?  One Mr. 
@Steve Wilkins