It's actually a very good question. In this video, a detailer discusses how the material of the body affects how much you polish.
[ame=http://www.youtube.com/watch?v=htsl7ICY8Nw]$1,000,000 Ferrari Restoration Detail - YouTube[/video]
He starts talking about it in depth at 8:40.
The entire video is worth watching, I think. Really great car.
Blackthorn One's above post is spot-on when it comes to polishing various painted-materials/substrates on vehicles.
-Not only is stamped-aluminum being used in place of stamped-steel (hoods, for example)...
But think of all of the different plastics/composites also being used.
-While weight reduction/better MPG may be some of the concerns for OEM's...
"Heat dissipation" while polishing, among other factors, are concerns of detailers.
I also agree…wholeheartedly…with Blackthorn One’s following assessment: