Why plane your intakes w/ milled heads???

normal practice.....i rather mill the intake face of the heads the corresponding amount so that the intake fits without cutting it...
Ive seen some do it the same way as my buddy, but it seems like Ive been seeing alot of milled intakes for sale. Which are now useless unless you have the heads to match. Just curious why people are planing the intakes instead of the intake face on the head. Good to see others are doing the same, and not ruining there intakes.