The problem is that C# or Db depends on the context. Not just the key you're playing in (although that's usually enough) but also where the note falls in the scale and what you're doing with it at the moment.
There is no reason to note a full measure of rest as anything other than a full rest. Two halves, four quarters, etc., does work out the same and "sounds" the same, but there's no reason to note it that way. That's the "joke". Only musicians will get it, and not all will think it's funny.
Edit: Okay fine, I can think of one time you might subdivide a measure of rest, but the notation would make it clear why that's happening and again it's all about the context.