Having had a couple of super-bright flashers, I won't ride without one anymore. I have had many motorists give me compliments on my taillights and say that they appreciate a cyclist making sure that they're visible. I've seen many cyclists at night riding with lights that were so dim that even after I noticed them they were barely visible.

As for multiples, IMO if you need one at all, you need two, because you won't notice when (not if) it fails. I guess it's reasonable to use something like a PBSF as the 2nd backup, but since really bright lights are so cheap these days, might as well run two of those.

As for horns, 90% of the time I'm just tooting "hello" back at someone who tooted and waved at me. But it has stopped incidents a few times, mainly when I'm coming down a hill in town at the speed limit (25 MPH) and someone starts to back out of an angle parking lot with a van next to them or something so they can't see. If I didn't have the horn I'd go wide of them, but that's dangerous too since sometimes when people back out of spaces they pull way too far out and I've even seen them go into oncoming lanes.