gnome-sound-appletisn't very useful in the Gnome Shell desktop environment, because the Shell has its own built-in volume control icon on the top panel.
The reason it's still around is probably just for the Gnome 3 fall-back desktop, where you get a two-panel layout similar to Gnome 2, which still has a decent system tray on the top panel. For Gnome 3 users without hardware acceleration, the gnome-sound-applet comes to the rescue to let these users continue adjusting their sound volume.
But, with Gnome Shell software rendering coming in Fedora 17, the need to maintain the system tray sound applet will quickly vanish, since even Gnome 3 users without hardware acceleration will be seeing the same shell desktop with its own built-in sound applet. This sucks a lot for XFCE and other desktop users. One of my favorite things about the Gnome sound applet is the ability to increase the volume higher than 100%, which XFCE's clunky old sound mixer applet still can't do.
"There are currently hacks in place in Gnome Shell that allows us to run the NetworkManager applet, but place its icon in the shell panel instead of the normal system tray. So, it makes sense to just program the NetworkManager to go directly to the Gnome panel instead, and no longer support the system tray version anymore. This is good for Gnome, and anybody using XFCE or some other desktop that will have objections to this change, well they can just switch to some other distro. We don't care."
On this one, I hope I'm wrong. But due to Gnome's short-sightedness with some of their recent design decisions (ruining Zenity, anyone?) I wouldn't be shocked to see them come up with such an idea.