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Many people mistakingly assume that to use Front Page extensions they must be on a NT host, that's likely the most frequent reason for wanting NT, next in line is familiarity with VB and ASP.
IMO though, *nix is much more flexible, and there are a huge amount of free resources for both the webmaster and the host, can't say the same for NT. You could always host both though, just keep an NT server set up and see if it is worth the hassle vs. the additional business you get.
The one case where I know it's crucial is when I've had dealing with FoxPro developers. These are some amazing database structures in this scenario that need an NT environment. Without looking, I'm not sure about ColdFusion - mivox may very well be right here.
Otherwise, permissions are a nightmare, admin is a huge pain and it is SO expensive.
Air is right. Frontpage can be hosted elsewhere, there are zillions of Perl scripts that can easily be installed on a unix box and your users won't be calling you at all hours asking for you to change the permissions on a certain directory or want to map some ODBC compliant database to your server.
If you are going to offer NT you should really map your costs of operation and admin compared to your Linux boxes. It is surely going to cost you more.