Microsoft is probably aiming to become a patent troll in the long run due to their lack of innovation. They already have many patents they can extort future innovators with, so picking up Nokia's (reported to be worth $6b) makes sense.
They didn't buy Nokia's patents. They bought Devices & Services business unit (the one making the phones) and a 10-year (non-exclusive) licence to Nokia's patents, which obviously are needed to make the phones. Does not sound like a patent troll move on part of Microsoft.
