What they should do also in the future (with Pirelli or other manufacturer) is to do a lottery on who gets to give their car to the tire manufacturer.
Giving a 13 car for a 14 season might not be ideal but a 14 car to test tires for 15 would definitely work.
So you have a cap with car manufacturers and the name that gets pulled out will have to assist Pirelli in developing the tires for the upcoming season.
They are never going to be able to decide who should give out a car and they can barely decide on a 2 year old ancient car.
A lottery would make things as fair as it can be for all teams.
Just my 2 cents.