I do it in a milling machine.
I think it was done to prevent mud clogging the fins, remember the B50, XL350 and XT500 where designed for street use not MX, if you look at a top-end intended for that use it often has more space between the fins.
A bit weightloss is a added bonus.
Anyway, I just do it to make it look like the photos of the original bikes from back then...