The Unity panel cannot be hidden because the code doesn't currently allow it. Why it doesn't allow it is because nobody has added the code to allow it. Is it written in stone that it has to be that way? I don't know. I doubt it. If you can come up with good solutions to the problems that arise when the panel is hidden, I'm very confident they will be considered. But it is important to not think about these things too subjectively. We can't consider one part of the puzzle at a time. We have to consider all of it simultaneously. For instance, we don't want the desktop to feel completely different if a window is maximized or not. That would be inconsistent and confusing.

The top panel is used for indications, and these are important. Because unlike other operating systems and desktops, notifications doesn't stay open. So you need a way to see that something has happened if you haven't been looking at the screen. This could be done differently, but then other issues tend to pop up.

The panel itself doesn't actually consume much extra space. Because it also saves space that was previously used by the window menu bars. So the screen real estate is just used somewhere else. If you have more than one window open, then it will often consume much less space than was previously used. In any case, on a large screen, the panel doesn't consume much space.

The smaller a screen becomes, the more likely it is that you'll maximize your windows and use one at a time. In that case, the panel doesn't consume any space at all. The window decorator will be merged into it, so even if it is still visible, it doesn't consume any extra space! Neat trick, that one.

In multiple screen scenarios, however, the discussion about whether it should be visible on all screens is valid and interesting. I don't know if any conclusions have been drawn in that regard. It might be that it should be configurable. But that's not an easy discussion either. There are so many possibilities, but we probably wouldn't want to have all those options in code and in config GUIs. Perhaps one good option would be to use the same choice as for the launcher? But some might not like that. My guess is that this will become configurable in some way in the future, but that it won't happen in 12.04. But that's just a guess.

I hope this at least helps explain why it isn't obvious that it should be possible to hide the panel. Or at least not obvious enough that it should take precedence over other tasks. And like everything else, you know, development resources are limited.