Interoperability, security, scalability and openness are some of the key requirements today's organizations are expecting from any platform to base their Enterprise applications on. Linux in general and Redhat Enterprise Linux (RHEL) in particular is emerging more and more as a preferred platform by companies of all size and shape.
RHEL is based completely on opensource components and hence does not have any server or user licenses. The cost that you pay is for structured support and maintenance and is fixed and predictable. More and more mission critical applications are being made available on Linux with every passing day.