I have an old and light cable lock from the 1970's. I doubt if anyone has that kind of key anymore and it will take a second or two longer to cut than a brake cable.
I use it for very short stops with my Brompton folders or my two lightweights in case I have to leave them in the street for a few moments, preferably when I have them in sight. If I plan a stop with any of these bikes I take a more serious and heavier lock, generally a U-lock.
My other use for the cable lock is to lock my rear panniers to the frame when doing supermarket shopping. My Ortleib panniers have the optional anti-theft wires (basically brake cables). Again, this won't stop a serious thief, but should deter an opportunist without tools.
The other family bikes have ring type frame-locks permanently fitted, these locks are fairly heavy and immobilise the bike, but a prepared thief could just pick the bike up and load it into a van and deal with the lock at leisure. I find a frame lock very convenient for short stops, but add a "serious-looking" chain lock (it isn't hard to cut with the proper tools) and/or a U-lock if I plan on leaving a bike for a significant amount of time, for example, supermarket shopping.
I consider locks a deterrent, not a real anti-theft solution. So I have old bikes that aren't worth much that I generally use when I know I will have to leave a bike locked up somewhere. And for some trips I use a Brompton folding bike and take it with me rather than leaving a bike on the street.
