Hey fillyjonk, I think reins can be overused, but they are absolutely necessary in some situations. Far better than a child always in a pushchair or (worse) car. I've never used them, but my two are far apart in age, and have always been reasonably easy to use. (I did own reins though, very good as a highchair harness when travelling to countries that don't use 'em, when you have a lemmingish child.)
The long boring explanation has its uses when they're bigger. We walk everywhere, my two are often on bikes (and DS2 has been this way since he was 2 at least), and although they zoom ahead, they do always stop.
My current amusement is, they stop at zebra crossings, and wait for me to catch up, fine. I'm trying to get them to stop further back from the crossing, so drivers don't get nervous (and to avoid the small risk that they trip into the road or something). But they've taken to stopping at the crossing, and then imperiously waving drivers on. I can see that it's really confusing to these drivers to take instructions from a 3-year-old on a wooden bicycle, but at least they can see he isn't about to ride out, and they do generally drive on.
I know they should stop further back, but watching them waving drivers on just cracks me up ...