Don't underestimate the impact that even someone using CentOS and not contributing back can still have. It's still a person or group of people learning the Red Hat way. Lots of people (myself included) started on CentOS, and later bought RHEL when the need came up because I was already super familiar with RHEL because of my years on CentOS.
Increasing proliferation of usage is a huge factor in driving growth.
Increasing proliferation of usage is a huge factor in driving growth.