If you live where fall or spring can have wide swings in temperature, you want leg warmers. If your ride starts with temps in the mid to high 40's, but you know they're going to climb up into the 70's or even 80's in a couple of hours, warmers let you peal off layers. I've climbed out of fog banks into sunny wheather with a 15-20 degree swing in less than an hour.

OTOH, knickers are great for overcast days that aren't that cold, but still require the extra thermal protection.