From what I understand Microsoft's goal is to become less of a software company and more of a devices and services company[0]. In that light, the acquisition of the mobile devision of Nokia seems to make sense.

And I guess it's pretty much the same strategy as Apple is following, especially now Apple has made Mavericks and several Mac OS apps free of charge.

[0]: http://www.microsoft.com/investor/reports/ar13/shareholder-l...