That's an incredibly powerful simplification in the right places. It's so satisfying when you can remove a bunch of "not"s from code and get something that's mathematically identical but a whole lot easier to read.
Heh! I also used it recently (and successfully) to argue with a particular vendor whose query language did not respect that transformation. I re-wrote a "not A and not B" expression to "not (A or B)", and the query broke. It was nice to be able to point them to the Wikipedia article and say "no, if your query language doesn't treat those as identical, then it's a bug and would you please fix it now?"