I remember when I first started using clipless peddles, you free the foot on the wrong side, and odds are high you are hitting the ground. Stupid easy to do.
Is it a certainty? Of course not. But, by that same argument, seatbelts are useless the vast majority of the time. Indeed, on most moving vehicles I'm on (busses/trains), I don't wear them. Not even offered in many cases. I still support and recommend them where they are available. Same for helmets.
If you're doing some sort of recreational/sport biking, then ya, a lot more people are going to agree you should be wearing a helmet.
Around campus, problems from not balancing with a backpack are more likely.
Though, I do mean to say I don't think it is a certainty. Just that it is more likely than you'd think. And helmets are good at low speeds. They are not just a high speed protection.