Thing is though, some people run smaller diamiter setup i.e small 195/50/15's and the car runs lower overall. Measuring from tips of ethier end of the side skirts to the floor is an easy measurement as you only need a ruler.
But most importantly shows me how far the actual car is from the ground as a figure, i.e saying 15cm rear/14cm front vs 34cm/36cm is much more informative & relative to 'Clearance'