I often submit equivalent queries when searching using Google. I tend to use OR a lot to specify alternatives to specific keywords (e.g. osx OR "os x" OR macos), though it's a years-old habit often not that important with today's search engines; I sometimes also use it to search sets of domains (e.g. site:w3.org OR site:whatwg.org). In general I feel that when the first few pages of results using a simple query don't get me to what I am looking for, using a more complex expression can unearth hits which I would have otherwise skipped or not reached.

Regarding Duck Duck Go's advanced search syntax, I've use it in a similar fashion, though not as much. Keep in mind that the provided link doesn't seem to reflect the current implementation. For instance, the syntax for grouping using parentheses doesn't seem to be as strict (which is a relief), excluded words are seemingly not limited to being at the end of the search string, and AND has a higher precedence than OR (which is currently a pet peeve of mine because I am used to Google doing it otherwise and I like not having to use parentheses in my usual queries).