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.
15 LibTech TRS HP . 15 NS Snowtrooper . 15 NS Ripsaw . 13 Salomon Pow Snake
15 Now Drive . 14 Now Drive
14 Flow Hylite