on my 5, which hinges at the front,
i didnt close it properly one day, and at 50ish mph the bonnet raised itself at the back, so i assume at that speed and over it will let air out, and again i assume its hot air as hot air rises.
so i would say, yes theyre functional.
im going to make a way of getting mine like it.

but if youve not got heating problems and dont like the look then take em off.