With most servers, though, it doesn't matter. The important bits (the server software) trivially fits into RAM, and the unimportant bits (the served content) comes off disk anyway.
That's way too general a statement to make. There is a great diversity of kinds of servers - different ages, scales, applications, etc., and few of us have a broad enough view of them to say anything is true about the majority of them.
Whether the important bits fit into RAM is the independent variable, not the dependent. The system designer chooses whether the important bits fit into RAM. So the only way the above makes sense is if you accept the oft-stated axiom that RAM is essentially free in 2012. I know there are servers where that is true, but there are plenty of servers where it's not. In one of those servers, if all the important bits fit into RAM, even though they're rarely accessed, that means someone screwed up and bought too much RAM.
The fact that you say everything but the server software is "served content," already tells me you're limiting your view to web servers and things like them. Other servers have very important data that is neither the server software nor originally from disk. If you don't buy either RAM or swap space for it, you don't serve.
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