That's not a very good comparison. Sunglasses are a small item that's trivial to store and carry around, it weights almost nothing and doesn't need to be parked

That's not a very good comparison. Sunglasses are a small item that's trivial to store and carry around, it weights almost nothing and doesn't need to be parked when you enter buildings. Obvious use cases for bike sharing is when you only need to ride a bike one way (so not a return trip) which happens very often actually (i.e. I sometimes ride a bike to a shopping mall but once I do my shopping I have to carry 2 full bags of groceries and other basic items so there's no way I'm riding a bike back, I'd get a cab). If you plan on walking somewhere for 20 minutes but once you're there you can take tube from there home as it's close to a tube station, another use case. People who live in apartments and don't have storage place to keep a bike. An so on, use cases are plenty which is evidenced by the fact that millions of people use bike sharing in Asia but also other places on a daily bases.