It seems to me that many using a guest account would not be knowledgeable Linux users. The guest account should be usable for most people not experienced in Linux. That is particularly the case as changes in settings only last during the current session.

The default Xubuntu desktop will in my view be virtually useless for many such users. The menu is hard to find, new users are faced with a panel they usually can't see at the bottom and won't be familiar with it so as to be aware what they need to do to see it and anyone without good young eyes will have trouble with the small resolution and dark on dark colours.

One will probably guess that I'm not crazy about the default desktop anyway. That is true, but for the most part it really doesn't matter to me as like most users I change the desktop. Still, I wouldn't recommend Xubuntu to someone trying Linux for the first time for the same reasons I don't think the desktop is suitable for a guest account.