I personally believe that if Mercedes now in hindsight had the choice of running either the private test they did with Pirelli or the Young Drivers Test (YDT) then they would choose the YDT. Their private test with Pirelli spanned 3 days only because of bad weather. They were using all sorts of random tyres and therefore any kind of information they could have gained would be very unreliable. Although at the YDT they would not be able to use their main drivers, Sam Bird is after all one of their test drivers so surely he will be a good measure compared to the simulator and they would get 3 full days of running where they know exactly what tyres they have on and can therefore do proper and reliable aero testing, etc.
The point I'm making is that for this season at least Mercedes I believe will have a net disadvantage compared to their main rivals as they are now missing the YDT, at least for aero, mechanical and tyre development related to this current championship. Whether they gained any kind of useful information for 2014, we'll never know. I do not believe Mercedes tried to "cheat" and I very much doubt Pirelli would be stupid enough to let them do so during their private test.
As usual it's the FIA that either within their own regulations or actions have caused another farce for F1.