First, it ferrets out earlyvangelists. They'll respond to your one line email with a book of suggestions and use cases. Treasure them.
Second, a non-negligible percent of your otherwise silent cancellations will get in touch with dealbreaker feature requests and support crises.
Third, your users with sales-potential will identify themselves by reaching out. If you email all your trial users, the ones who are seriously considering a purchase will jump at the chance to talk directly to the CEO or founder.
We used this approach in the early days of Woobius, and it certainly helped. As discussed previously, early on, metrics (and even A/B testing) don't make sense because you don't have much data. These types of approaches help you get some kind of insight at the point in time where statistical significance is still a long, long way away.