I agree, I wouldn't be surprised if the weights people manually assign to the questions have very little correlation with the success of their relationships. People probably suck at knowing what they want.
That's not to say they're meaningless though; e.g., if someone puts "mandatory" for all his questions, that definitely says something about his personality, and should be used as a feature in the ML algorithm used for matching.
People may suck at knowing what they want, but I think it's more that people do not necessarily know what criteria they should want to select given their goals for finding someone.
Another problem is that a lot of users are probably more casual about the weights, such as selecting mandatory. (It's common for people to select an answer and then mark that same answer as unacceptable in a match, for questions in which it makes no sense for them to do that.)
That's not to say they're meaningless though; e.g., if someone puts "mandatory" for all his questions, that definitely says something about his personality, and should be used as a feature in the ML algorithm used for matching.