Voting in November makes the sense because it will bring more voters into the process. The hardcore people who vote in the May elections (and it's what, 5 percent of the eligibles?) will be supplemented by people who only vote in big elections and it will make a difference in the quality of candidates and the quality of the message.
I don't believe the races or the coverage of them will be diluted; quite the contrary, the local election will be amplified. The Republicans, of course, will oppose anything that will bring out more voters and this will. But I don't think there is a Republican on council ... in name anyway.