Well, the post was asking "which windowmanager" not "should I run a windowmanager on a server?", so I figured I would try to answer the question, instead of argue the basis of it. I think this question didn't just mean servers, anyway.
Yes, usually I don't run X on a server, but sometimes (especially during development work) I like to have multiple terminal windows open, and sometimes I like to run those cool resource tracking widgets, just to see what's happening.
IMHO, X is only an insecurity factor if you don't know what you're doing with it. Also, as i said, before, if you only fire it up when you are actually using it, and turn it off most of the time, then what's the problem? When I use it remotely, I use SSH tunnelling, so it's about as secure as you can get.
So in short, IF you are going to run X on a server (and I'm not saying you should), use a lightweight like Blackbox, and otherwise, on a workstation, Blackbox is great because of its speed.