It's most likely a talent acquisition, not a product acquisition. Keeping Summify up and running would be a waste of developer time (from Twitter's point of view).
With such a long list of investors (http://www.crunchbase.com/company/readfu) it must have been quite a large sum of money to acquire Summify. So IMHO not just a talent acquisition.
I guess there are some cases (like Wordly ----> Google Docs) where that happened and was good in the end?
Most seem to mean the product is completely dead.