(Disclaimer: I work for Red Hat on virtualization)
Red Hat and Fedora have a strict "upstream first" policy. We also have a large team working on KVM and qemu. A natural consequence of this is that we implement many features and fix many bugs in KVM/qemu, and these go upstream, and every other distribution benefits. This is great for open source. But I think your question is How is it good for Red Hat? since your implication is you can free ride on Red Hat's efforts.
There are three cases where you might benefit buying RHEL: Firstly if you call support with a serious bug, then eventually it'll get escalated likely to the person who actually wrote the original code. Secondly RHEL subscribers influence the future development direction (of course, the larger ones have a bit more influence). We really care about how our customers are using the tools. Third, you're probably not just using a single KVM host, you might want to try out OpenStack or oVirt, and we have systems architects who help customers with these larger deployments - the same architects who previously worked with large telco subscribers using OpenStack or huge bank deployments of oVirt, so they have loads of real world experience.
However if you're happy to free-ride, then us developers are happy too, because at the end of the day we really care about Free software.