I run a mixed environment right now with OpenBSD, FreeBSD, Red Hat, and Windows. I use Red Hat and Windows because certain software requires it (basically they are single app servers[1]). The OpenBSD servers are all doing basic utilities and the FreeBSD servers are doing stuff the requires a big file system (ZFS). There are a lot of enterprise tasks that OpenBSD is fine with doing, and I just use FreeBSD for tasks that require the file system or really heavy load.
1) government contractors are so fun when they get their software required to deal with certain parts of government