For the sake of this example, let's assume you have customers that depend on your server being up, for the product/service they're paying you for.
Tell the customers that you plan on rebooting once a month, and see how they feel about it.
Being in a situation where you're completely unable to restart a machine without losing money, is being in a situation where you're living on a knife edge regardless. A single hardware failure and you're in deep trouble.
I've been in exactly that situation (the entire company rested on a single Ubuntu server), it was really unpleasant and management didn't do a thing about it until we started getting sector errors which the RAID controller's error handling was struggling to cope with (after that incident and the downtime, management shelled out for four physical boxes, a SAN, VMWare licenses, and a setup with 15 minute rollbacks and dynamic migrations if a physical server died).
No, it isn't untenable, yes you can work around it, but that's a problem that shouldn't need to be worked around.
Maybe Linux doesn't need to restart the OS as often, but you're still going to have down time, right?
https://www.youtube.com/watch?v=p_vUZR2RtRs
https://www.youtube.com/watch?v=xkKoggifsE4
(And you don't need to log in for your background services to start.)
Worse, the conversation was actually about the time needed to install updates. Now that Windows logs out before installing updates, you have a fairly sizable time delay – tens of minutes after a large update even on an SSD – while the service is unavailable but before the system starts rebooting.
How does Fast Boot work? Well, it's actually not booting at all – it's logging out all active users and hibernating:
http://blogs.msdn.com/b/b8/archive/2011/09/08/delivering-fas...
That's a really nice bit of work in normal usage but it means that you have to do the standard cold boot process when you're getting a system update. That means that a sysadmin would need to review each update to know whether it would trigger a fast or slow boot, assuming that a patch for e.g. SQL Server wouldn't always trigger a cold boot to be conservative.
The alternative is what everyone's been saying, namely that you have to assume that a reboot is slow and have n > 1 servers if uptime matters.
However, if people are going to make irrelevant points about "not having downtime with Linux" then I am going to fuck with them.
BUT if Windows patches came out as often as CentOS patches do, I'd been rebooting every other day.
Look it up, it's awesome. (Please don't.)
Plus I can schedule this restart at any hour, on any day, I wish. And do. For example I restart mine on Thursday morning at 1 am.
If that really makes Windows Server "unsuitable for servers" then your downtime requirements must be so high that any single server wouldn't be able to meet them (i.e. even with Linux you'd need two or more servers to provide that level of uptime).