This is getting to be very frustrating. The fact is that economists use surveys/questionnaires all the time. The problem is we hardly know it since we don't collect the data ourselves. The CPI is a survey, expenditures are determined by a survey, and prices are collected by a survey. All subject to the same social desireablity bias noted in the post. IRS data, BEA, census? All surveys. Surveys are not on their face good or bad. Yes they are prone to bias as the previous comment notes, but how would you have us gather that data otherwise? Follow someone around with a camera 24/7 making note of who they sleep with?

The important thing is to try to mitigate any bias, and understand when the respondent is not likely to know thier motivations. Simply deriding the method is not so useful.