DDG actually pulls its results from Yandex and several other sources. Why limit yourself to a single, old-fashioned crawler, when you can use one of the most innovative meta-search engines?
Meta-search is a pretty old idea too. And you're at the mercy of your sources and have less insight into the data you're searching and less ability to rank pages.
http://help.duckduckgo.com/customer/portal/articles/216399