it reduces the chance of deliberate theft by the service.

One problem with bit coin, is that an exchange service can suddenly find itself with millions of dollars worth of bitcoin, dwarfing the possible income available via fees and start thinking really, really hard about where the most money is to be made.

and then the service might go out of business, and the customers lose their property.

Not to mention the target the service might represent for hackers etc.

Having a dual key system ensures that, for all parties, the best possible outcome of attempted theft is nothing at all for anyone.

Not a great outcome for any specific customer, but an outcome that aligns the motivations of the service provider and the customer.