Of course you may be right that Putnam is a pea in MMC's pie. But, the other side of the story is that MMC has historically carried a premium P/E multiple compared to the other competitors. The reason - again historically - is that Putnam was viewed as a much faster growing company than the rest of MMC, which resulted in a higher multiple for MMC. In fact, up until around 2002, MMC deserved its premium P/E becasue Putnam, in fact, caused the entire company to grow faster than the competitors.
Now, for the forseeable future, Putnam will HINDER MMC's overall growth and, in the same way that Putnam's superior growth in the late 90's/early 200's caused MMC to have a premium P/E, it is logical to believe that Putnam's drag on MMC's overall earnings growth will result in a below-average P/E for MMC.