I broke one on a BBS-HD bike, but it was my own fault. I replaced the rear cassette with an internally geared hub and as part of the process, I replaced the chain (or maybe I just shortened the original). I thought chain links could be removed with a chain breaker, but apparently, chain technology has changed. The chain was OK for a short while, but it broke 6 miles from home on a rainy day. Once the chain was replaced and installed properly, I had no more issues.
Chain wear can be measured and the chain should be replaced before it gets too worn - much like a timing belt in a car. But I'd expect chain life expectancy is influenced a lot by the conditions the bike is ridden in and the diligence with which the owner maintains the chain. Suffice it to say that a broken chain can ruin your day, so check for wear regularly and replace on time. +1 for hub motors that won't put undue stress on the chain and will get you home even if the chain does break.