For me, "beater" has nothing to do with bad vs. nice weather. It has to do with where I'll be locking it up and for how long.

I'll lock the beater ('88 Trek 400, salvaged from a dumpster, $100 invested in the repairs) downtown at Pike Place and not worry about it.
There are some control stops where I bring my brevet/commuter inside the store with me because I won't leave it out of my reach for 2 minutes.