According to this report: http://www.businessinsider.com/microsoft-was-interested-in-m..., Microsoft was not interested in buying Motorola. Instead, they were (supposedly) close to settling their patent dispute with Motorola, and signing a cross-licensing deal giving them access to Motorola's patents.
If Microsoft was settling their patent dispute with Motorola and looking at a cross-licensing agreement, that is all the more reason for Google to buy them. Either Google gets a cross-licensing deal and gets the protection they want for the Android ecosystem (at least from Microsoft) or Google winds up with a patent portfolio that Microsoft has already decided is dangerous.
