While many might feel that these suits are a waste of money and time, as a public company Microsoft is compelled to protect its intellectual property which in theory, belongs to the stockholders. As the company said in the press release, "We have a responsibility to our customers, partners, and shareholders to safeguard the billions of dollars we invest each year in bringing innovative software products and services to market. Motorola needs to stop its infringement of our patented inventions in its Android smartphones."
Even though Android is an open source system, Microsoft has been able to invoice HTC for using the OS. So far, there has been no response from Motorola.
source: Nokia via Engadget