This is probably something you can even apply numbers to. Let's say you're a database company. You sell support on a yearly basis.
Upgrades require a new purchase.
You also offer classes. And finally, you've got a growing but healthy consulting group who implement your database for customers. At first glance, the suggestion that you give away your database software for free would be ludicrous. This of course assumes nothing else changes in the equation. But the chances are, if you pull this off right, things will change.