Microsoft lives off of what Oracle, RedHat, HP or even IBM live off - commercial support. To large corporations, having someone to blame and who can vouch for or guarantee fulfilment of signed contracts means more than anything else, because it gives them legal way to offload responsibility and any legal or financial ramifications in case of a mistake. Windows owning the desktop operating system market, for now, is a clear choice for such services. So it depends on your needs rather than general question for value proposition as that depends solely on your specific requirements.