It makes perfect sense, they've got some technology out of it, some people who understand mobile, and a nice big headline. Worth $30m of my money? Probably not. Worth $30m of Yahoo's money? Maybe, they're not likely to have decided to spend that much on an acquihire without board approval really are they. Unless they Zuckerberg'd it and thought it was better to seek forgiveness.
The summarization tech isn't owned by Summly. It's owned by SRI International and was licensed to Summly. Maybe a large chunk of the reported $30 million was actually used to acquire that technology from SRI?
[1]: http://www.forbes.com/sites/parmyolson/2011/12/13/teenage-pr... [2]: http://techcrunch.com/2011/07/15/trimit-summarizes-emails-bl... [3]: http://gigaom.com/2011/12/13/meet-the-internets-newest-boy-g...