It was certainly done during the GW2 beta. It makes sense. If you don't shut the forums down, the flow of comments never, ever stops. At a certain point people aren't reacting to their experiences, and are just reacting to each other. This often turns unpleasant, and at that point the information provided is useless.
Generally beta feedback is specific technical issues, specific bugs, and overall impressions. The latter is honestly easier to process by way of a survey with quantitative data that can be processed and anolyzed, not by long, written opinion pieces. Sampling some of those is useful, but the greater the distance between the event and the feedback, the less useful it is. Customer feedback is a well known and well-researched field, as are data metrics; based on decades of studies, this really is the logical thing to do.