mclaren built their cars since 2007 to hamilton's preferations. so no, not just 2012. years later, button comes in and gets some input in the car so the 'new' car has some ability to 'dial in' more to button's preferations. and i didn't say button was better then hamilton, but the fact remains he was 2nd in the WDC despite a car that still was designed for their star driver hamilton. alike hamilton has to deal with a 2013 mercedes which was essentially built to rosberg and schumacher's desire.
the 2014 merc will be 'adapted' more to hamilton's desires.
alike this year, sergio entered a team with a car that was built and designed around hamilton and button before.
If sergio would have stayed longer, mclaren would have done effort to 'adapt' the car in a manner that sergio can dial into
the car better because of his 'driving characteristics'.
let's say one driver A wants a pretty nervous car, and driver B wants a smooth car.
if driver B comes into team A, built around driver A, he'll have to adapt because the car was finished when he stepped in.
a year later, driver B has a updated car that before was nervous and now is smooth[er].
and vise versa.