My philosophy is that, if it helps keep me safe, it's probably worth the investment. I'll never know how many collisions I've avoided thanks to my mirror, reflectors, and lights, or how many broken bones I've prevented thanks to my studded tires, but I'll never regret the extra outlay. By the way, my tire sparx were still working today... -35C.
Even with my best efforts, I usually have to take evasive action a few times a week when a motorist doesn't see me. It's dark, they're cold, and they often don't do a very good job scraping their windows, especially the rear, since the defroster will take care of that eventually. Then, their first maneuver is to back out of their driveway/parking spot. Plus, I definitely experience that 'safety in numbers' effect in winter; when cyclists are rare, nobody thinks to look for them.