Actually I think you might be right in that Gnome assumes that if you disable ESD that you have lost the ability to do sound mixing and therefore disables all the system sounds.
Certainly on my system if I uncheck the Enable Software sound mixing (ESD) all the system sounds selections in the Gnome Sound Preferences Panel become greyed out. This doesn't really affect me as although I don't use ESD, I don't want the Gnome Sounds enabled either
I can't remember reading your original post but have you checked that the default sounds are actually present in /usr/share/sounds and/or have you tried loading a sound file that you know plays with esdplay. Do the previews work ?
An interesting side effect on my system is that although ESD is not enabled and or functional as far as I can tell, If I check the use ESD option in sound preferences then the sound chooser applet will allow me to preview event sounds but they won't actually work for events. Except the login and logout sounds.