For the last decade, I've regularly seen complaints when we harder-core software freedom advocates spend some time criticizing proprietary software in addition to our normal work preserving, protecting and promoting software freedom. While I think entire campaigns focused on criticism are warranted in only extreme cases, I do believe that denouncement of certain threatening proprietary technologies is a necessary part of the software freedom movement, when done sparingly.
Denouncements are, of course, negative, and in general, negative tactics are never as valuable as positive ones. Negative campaigns alienate some people, and it's always better to talk about the advantages of software freedom than focus on the negative of proprietary software.