Great article. Designing a/b systems always seems (relatively) simple at the start, but in my experience there are 1,000 things you don't think of until you have massive amounts of worthless results. Add this to the list of things to watch out for.
I think using them together is crucial. Qualitative research gives you insight into what you should look for through quantitative research. It's important to start with some basic qualitative research (talking to a few customers) and then scale it through quantitative methods (analyzing logs).