I did hope that this might turn into a 'reasonable' debate of where McLaren are now and where they would need to go in future, not just as a racing team but as an organisational whole (having an effect on the racing team), but this whole FOTA thing has taken this way off-topic and isn't the slightest bit interesting.
KERS certainly gave McLaren an advantage last season, and they certainly chose to (sensibly) maximise the advantage they had there because it was something most teams didn't have and they had to make something of the thirty odd kilos they were carrying. That's culminated in the F-duct this season. If they were to use KERS with it then it would have added up to a pretty reasonable advantage because the F-duct would have multiplied the effect, but KERS was a pretty costly off-shoot of development that would have delivered little advantage that they could have held on to. The cost/benefit just wasn't there even if they had turned their back on FOTA after everyone agreed it was too costly to develop their own systems. That's why the gentleman's agreement stuck with KERS but didn't stick regarding the sneaky F-duct system.