Depending on the types of cars, theft is a valid concern. HiGear was a car service that rented out luxury cars and had to shut down because they were hit by a criminal ring [1].

Agree that its not a showstopper if you're vigilant, so it's good that they're thinking about it from the outset.

[1] http://techcrunch.com/2012/01/01/luxury-car-sharing-service-...