It's not very common for bicycle riders in the US to explicitly carry insurance. Your homeowner's or automobile insurance would typically cover the value of the bike and any damage you might cause in an accident.

If you are banking on your homeowner's policy you should check the small print. Many (most?) have a relatively low cap (say, $1500) with a deductible of $500+ you don't have much coverage. My insurer requires a specific extended policy that at $3 per 100 makes it very expensive for higher-end bikes.

Pay attention to this if you have really nice bike(s),. They aren't going to be covered under your homeowners in the U.S.. Just like jewelry, expensive camera equipment, etc, you'll have to get a specific insurance rider to establish value, etc. and they'll charge you a nice premium for this.

I wouldn't think many people would consult a registry in order to assure themselves that the bike they're buying isn't stolen; rather, they'd use it to find a bike they already owned and that had subsequently been stolen, as a means to try and find it. I suppose a Pawn shop or similar would use it for a purpose as you suggest, though.

Why would a thief register his stolen bike online after the fact? I think you've got this mixed up, these registries only work when people consult them before buying.

I don't think he ever implied that a thief in particular would register the stolen bike. The first scenario is a buyer checking beforehand to see if it was stolen. The second scenario is the owner of a stolen bike checking to see if someone else has re-registered the bike, like someone who purchased the stolen bike thinking it was a legitimate sale (a bit misguided, as the system doesn't quite work that way).

No, dealers consult the registry when required by law, and police consult the registry when they recover a bike.

Most buyers don't care, they have no incentive to care.