At the time that most teams agreed to remove KERS, KERS cars werent performing great. In fact, at this time, I remember Ferrari's KERS system was causing serious problems, forcing Kimi to run without it. I remember BMW soon abandoned it. KERS wasnt great.
Fast forward a few months and KERS is now proving a fantastic weapon to have. Red Bull cars, as fast as they are, have 2 frustrated drivers who have no KERS and are making comments to suggest that they would like to have KERS, as they are getting battered on the long straights and start-lines.
I don't believe that any team (or principle) would willingly give up a weapon that is working so well and is helping move their cars up the grid, once the lights go green. If that principle is willing to forgoe performance, just so he is liked by other teams, that man should not be in charge of the team.
You have men in the factory who are working their backsides off, night and day, long shifts, not seeing their families/wives, etc, just so that their cars can gain a 0.1s/lap advantage and the principle is worried about a "genteleman's agreement". I for one, would be livid if I was a worker at McLaren's factory and they axe the KERS system, which I would've spent many hours developing, especially when I know that the KERS device is not illegal or banned.
I don't think McLaren will cry at losing KERS, neither will Ferrari.
Look at it this way, McLaren are using the same KERS they started the season with, it's not like they have changed that drastically, but the car itself has got much better. It is the same with Ferrari, they are better now without KERS than they where with it.