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There is a feature in XFCE that allow you to force a margin around the desktop, so that windows can't cover it. I think it's useful because with a big margin at the bottom or at one side, you can put a floating macOsX-like dock or some other plasmoids that will never be covered by windows.

Moreover, when someone use a very high resolution for his display, maximizing windows is not very usefull because the maximized window is really too big (and it just looks better with a margin around the window ). Finally, I'm a big fan of the "switch desktop with scrollwheel" and with this feature, I can always have some visible area of the desktop.

Actually, I had already began to implement this idea as a new desktop containment. Here is a short thread about it :
http://osdir.com/ml/plasma-devel/2009-04/msg00009.html
Following advices from Plasma devs, I added a strut to the default desktop containment. It basically works but I'm not very experimented and I had two problems :

- Plasma popup menus (kickoff, calendar...) respects the strut and so they are opened below the margin
- When maximizing a windows, the decoration is not painted so you're ending with a borderless window in the middle of the screen

So if someone think it's a good idea, feel free to implement it