Here's my problem with it. Newbies come in here, read the posts and come away thinking that commuting by bike is fairly complicated and requires a lot of gear:

spare tubes
fenders
lights
multi-tool
tire levers
pump
rack
panniers
rain gear
etc, etc.

Now we're going to tell them that a dedicated commuter should really have a spare bike?

I'm sorry. No. We don't have to make it that hard because it's not.

If you have the sort of job that you simply can not afford to be twenty minutes late one day, then you can't afford to awaken late. You need to have a plan for what you're going to do if something goes wrong. A spare bike is one option of course but it does you no good if something happens while en route, - which is more likely anyway.