You make fair points, tjspiel, but my opinion remains as it is. I didn't -- and won't -- use the word "necessary," because I think it helps to have the skills, but I can't claim it's necessary. Most people have a way to bail if the bike stops rolling. But it's more pleasant to get oneself rolling again.

Thursday, I broke a spoke, and my wheel was so wobbly, it wouldn't clear the chainstays. I had a multi-tool which had a spoke-wrench-like thingy. It wasn't efficient, but I managed to loosen the spokes adjacent to the broken one and get the wheel rolling again. Now that is an advanced skill which I wouldn't implore everyone to develop, but it certainly was better than carrying the bike to the subway. My bike is pretty heavy. Now, you could argue that that kind of breakdown is rare, and that would be a fair argument, so I guess my point is that the more skill you have, the more chance you won't have to bail. Bailing is unpleasant.

You want me to list the things that can go wrong? The problem is that there are many, many things, and no one thing is particularly likely, but it is very likely that one of them will happen before too long. My daughter's roommate's derailleur failed because the cable housing frayed enough not to "house" the cable any more. I don't see that often, but hey, it happened. Brake cables snap. Tires blow out. All kinds of unlikely things happen, adding up to a likely event of one kind or another.

I am not professing that everyone should become as skilled as I am. I'm saying the more skill you have, the better.

There is a great treatment of this subject in the book Zen and the Art of Motorcycle Maintenance.