You are forgetting that Nokia as a company has done a major share of R&D in mobile space since 90's, thus they have huge chunk of patents covering the fundamentals. Now that their phone business is suffering they have to monetize somehow, and that how is their IP portfolio.
It would be irresponsible of Nokia not to sue, as their shareholders want to see money.
Most manufacturers have licensing agreements with Nokia besides some of the new comers such as Apple and Google and Apple already paid them off after getting sued.
Now there is no question that Nokia fucked up. The fuck up was mainly caused by internal politics where different division screwed with others. I mean they had a mapping company bought years before google but they only got to market a year after them.
Shit happens and now they've only got the low end and what ever IP they've developed. As they have to rebuild their credibility on the high end they need some cash to cover it, as such IP is the best source.