Monaco does have straights where KERS may be an asset in attacking or defending. It's just that almost every corner is so darn tight. Kers just does not provide as much a benefit as other track configurations.
There are many reasons for McLaren to retain KERS. First off, if KERS was to be ripped out, it is for balance purposes. KERS comes out, and the entire car would be changed by re-ballasting. I don't even know if McLaren have run their current car in such a configuration. Any major change in balance would be a LOT of work.
It's not about the immediate situation at Monaco, but more about long range thinking. It would take more than just removal of KERS to make their car a potential winner. But by retaining it McLaren adds to a huge database on KERS that other teams do not have. And notwithstanding all the present politics, next year McLaren would enjoy a major advantage because they have run it (and learned) at every track. KERS will be mandatory next year, and what they learn today will stand them very well in the future.
And if KERS becomes a spec component next year, like the ECU, McLaren look likely to land that contract because of their success and experience with KERS.