To avoid the hassle of ever not having one when a lifty decides to actually enforce the rule...and it's happened many times. From experience, I know how much it sucks to have either hike back to the car to get it, or wait in line forever and shell out $10 at the rental shop. I keep one in my pocket as often as I can remember.
16 Gnu Swallow Tail Carver . 16 NS West . 15 NS Snowtrooper BIA
16 Burton Genesis X . 15 Now Drive
16 Flow Talon . 14 Flow Hylite