Because more than one notification area doesn't work, and because the active one sometimes becomes invisible (for example when you change resolution of the screen and a panel disappears, which appears to be a known bug in gnome-panel): the user will think that his notification area doesn't work anymore.

Two reasons to make it a singleton.

So either only one notification area should ever be visible and/or created or it should be possible to set the currently active one, all of the existing ones should show all of the notification-icons or the gnome-panel bugs desperately needs fixing.

Another bug: there's no notification-area component.