Just after DrupalCon Sydney at the start of February of this year, I overheard some people wondering why they should sponsor a DrupalCon. Considering the people who attend, there's not a lot of product selling you can do if you're a Drupal shop and unless you're looking to hire delegates as new staff, there's not a lot of direct benefit from having a sponsor booth or table.
Obviously, helping to fund a DrupalCon and the Drupal Association via a sponsorship are good things to be doing for the community, but the payoff isn't necessarily immediately apparent. However, there definitely is one. There just hasn't been a metric for it, let alone a testable metric.