The only true reason I can see to use a Windows OS is to have a dedicated computer at your side, because linux isn't made to handle the same kind of wear and tear that windows is.
No you're right, linux will take alot more wear and tear than windows. Host and ISPs use linux for uptime and flexibility, not just for price.
The cost of a windows license, is nothing in the long run, but why pay for Windows if it's not necessary as Linux is better as most things than windows. (.ASP and .NET are the exceptions)
Free is not an issue for us, we pay monthly fees for our Supported Enterprise Linux, as we would rather have support for our customers rather than just community support.