That would seem kind of redundant as it's already not necessary to answer every question, even the ones that don't say they're extra credit or skippable. Maybe Yvain could have made that clearer at the beginning?

I personally wouldn't have minded a longer survey either. I'm just worried that making it longer would deter others from completing as many questions or even taking the survey in the first place. It might be a good idea to have a poll (perhaps within the survey itself) asking for the amount of time we'd be willing to spend on such a survey.