Moody's Investors Service said Wednesday that it will start rating municipalities on the same scale as corporations and countries, a move that may reduce demand for bond insurance. The move could also provide some relief to money market funds and other investors that have to hold top-rated securities. Laura Levenstein, senior managing director at Moody's, said the agency recently decided to assign ratings from its corporate and sovereign system to municipal issuers upon request……. If a lot of municipalities take up Moody's on its offer and get upgraded to AA or AAA, that may reduce demand for bond insurance….. Shares of Ambac Financial, one of the largest bond insurers, fell 11% to $6.86 on Wednesday. Rival MBIA Inc. dropped 4.9% to $11.55.