I don't own a pair of leg warmers but I can see their value if there's going to be a temp swing during your ride that would make basic tights too warm or shorts too cold. They're not enough for winter riding in the Upper Midwest.

There are a lot of winter clothing options that do work though, -including a pair of wind resistant track pants with ankle zips worn over a base layer and regular pants. As long as there's not a lot of excess material below the knee you're good.

I usually wear my Craft XC skiing tights over bike shorts or another layer of tights. If it's really cold, I'll wear PI Amfibs over padded tights but I like the Craft tights better. I mentioned the track pants because thats what I do when I'm riding a short enough distance to wear regular clothes.