When user testing is performed, users are often presented with a serious of variations of a product and observed to see where they excelled and where they had trouble. People will often click in areas that are not intended to be clicked on, or may try to achieve a goal in a way that you had not designed for. Now put yourself in their seat for a moment. When you go to click on something, why are you doing it? You are doing it because either the interface has made it clear what to do, or you are simply relying on how you think things should work.
Once the results are gathered and analyzed and conclusions are made, the research is published and becomes factual information that others, like ourselves on this site, use to help create their products and interfaces.
So step back for a moment:
Isn't all user testing essentially a way of gathering how users think something should work and then making conclusions from that research? (seeing as all decisions that are made while being tested are based on how the user thinks the interface works
If so, then how are what ones thinks of the best way to do something (like on this site) often only taken with a grain of salt when they aren't backed up by user testing/research? Yes it is very true that using research to support your claim strengthens your argument and essentially says "I believe this and 100+ people in this study agree with me". But how is this any different that 100+ people agreeing with your individual opinion that is not backed by research?