Sometimes it comes down to simply how you feel about the control of your own future.
Working directly with Red Hat puts you in an important position, a paying customer, with the people who are experts at getting problems fixed and new features included in upstream projects.
Red Hat is the catalyst between you as a customer and many thousands of upstreams, not just the Linux kernel. Take a look at some of the past and ongoing contributions that have come from Red Hat, some of which were influenced by customer requests:
When you use a rebuild of Enterprise Linux, you gain the same benefits flowing downstream that Red Hat customers gain, but you change the dynamics of how you can manage your relationship with the upstream.
Is your in-house support staff prepared to negotiate with upstream projects on your behalf? Work to get patches and features included in the code?
If they are, that is great; the world needs more corporations that are contributing from within their own IT staff. Again, though, when doing that, having a partner, such as Red Hat, to work with on negotiating through the upstream maze can be helpful.
People who work to produce Enterprise Linux rebuilds are in the same position with regard to how they can influence the upstream. If they are a paying customer, they can ask Red Hat for help. If not, they are welcome with everyone else to participate in the upstream project for RHEL, which also considers itself the upstream for CentOS, Scientific Linux, Whitebox Linux, and the myriad other rebuilds:
For a long view on how and why this works, I encourage you to take 9 mins and 21 seconds to watch Michael Tiemann talk about "The Open Source Triple Play":