I'm not saying that mirrors in themselves are poo (once you can see past your elbows,) but I am saying that some riders become complacent with their use (by your own admission you've been "tooted" for not looking behind before moving lanes etc,) and can end up relying too heavily on them.
We've been designed with a swivelly (is that a word?) neck, a perfect bit of kit for looking behind you, and that has been proved to save lives, it's even called "the lifesaver."
I was merely offering a counter argument to the posts that were telling the poor chap who asked the original question not to bother fitting mirrors because they never had and never would. As I said before, I also look over my shoulder where confirmation is needed but that's usually just confirm there's nothing in the blind spot. The tooting btw was when filtering in 3 lanes of traffic with cars practically bumper to bumper. I knew there was a bike behind to my right but didn't reckon on the one sneaking past on the near side. I only had one mirror on the scoot then 