I only carry a spare in situations where there's no other backup plan. These days with uber/lyft if I ended up stranded I can pay to get picked up without it being horribly expensive. Half the time I could still bike home with no front light, just a little slower and more carefully.

With mountain biking it's very different, after some "how the heck am I not back yet, am I going to be stranded??" once, I carry at least 1 backup light. The Fenix BC21R is the smallest (counting all the parts including the mount) backup light I've found that puts out decent light:


Battery if a standard lithion size and user replaceable, don't have to replace the whole light to replace the battery.