Customer cars are attractive in the respect that they reduce the costs for the customer and amortize the cost for the constructor. It does mean that, in all likelihood, the customer will get a 1 year old car. Mind you, Force Inida would love to have a 1 year old Mercedes right about now.Allowing customers to use the same spec cars as the constructor would be asking for trouble among the smller teams that choose to remain a constructor. A 2015 Williams shoudl be close enough to a 2014 Mercedes, but if two teams have the 2015 Mercedes expect a lot of complaining from Williams.
Another option is to allow customer teams only one car, rather than forcing two car teams. Their operations can be smaller and more focussed.
The problem with 1 year old cars is, of course, rules stability. Of which there has been precious little of late.
This could be solved by the price of a customer car of course. A 2014 Mercedes car should be more expensive than let's say a 2014 Williams car. And of course, customer car is a big word with many variables available. Many things can be adapted to a customer car. A team who wants to buy a customer car only get a chassis for example and has to develop the rest by themselves.