We wish we were all getting paid to do this, and were getting followed by our directors sportif in Audis with spare bikes mounted to the roof. In reality this isn't the case, so we need to be prepared for the inevitable mishaps that are going to happen when you ride a bike over road debris for 90 miles at a time.
Minimalism is the acme of the competitive cyclist, but worse than being the dork with the seat bag is being the dork who has to borrow (and never return) a tube from the guy who was smart enough to bring one.
A (very small) seat bag is acceptable, indeed recommended, if you are riding without a support car. This seat bag should have at least:
1 tube. ( a new one please, carrying along patched tubes is embarrassing. I'm embarrassed to be with you if you're too cheap for a $3 tube.)
1 tire lever (1 more is acceptable)
1 inflation device (CO2 or a micro pump)
1 small multitool is acceptable but not necessary.
That's it. Your seatbag should look like a small, tight pack under your seat, not a huge swinging testicle. You don't need another tube in case your first replacement needs replacement. You don't need a spare tubular (triathletes do this, and they don't know how to change tires in the first place... odd. ) You don't need a new set of spokes, a torque wrench, or a frame pump. Your goal is to be able to get yourself home with the most likely mishaps taken care of, not to be a rolling bike shop.
You do NOT need a mirror of any kind. Not on your helmet, not on your sunglasses, not the impaling weapon that mounts on your handlebars. If you are incapable of turning your head or using your ears to get a sense of your surroundings, stay the fuck away from me and any other bikers you see.
If you are riding with somebody who doesn't know the rules, you should give him(her?) your spare tube, but feel free to be as smug as you can about it. Don't expect to ever see your tube returned, but you can hold a grudge if you'd like. As with any of the rules, just make the offender feel as awkward as possible, and you're doing the right thing. We all thank you, good sir (madam?).