Allthough mathematically 1 ∈ all, it really only makes sense to enable "all" if n > 1. Else, we don't need "save".
Imagine the following situation: You have a lot of editors open. One of them is modified (or maybe you're not sure if any are modified). Unfortunately, it's not the current one, and the modified one is not visible in the row of tabs at the top of the screen. You now either have to scroll the tab list
1 or use the open files list
2 to get to the tab and hit "Save".
Wouldn't it be easier to just hit Ctrl-Shift-S to "Save all"? Wouldn't it make sense for that option to be enabled then?
Besides, I've never seen a multi-document interface that didn't allow you to "Save all" if there's only one document modified. (I've seen some take the easy route and never disable "Save all", though)
Of course, I might not even notice, since the only time I ever need to save anything is right before a build, when it happens automatically

.
1) And possibly guess which way to scroll if both directions are possible.
2) Which is rather small, at least for me. The project view is usually more important to me.