You can turn it on with
gconftool -s /apps/metacity/general/compositing_manager true --type=bool
or by running gconf-editor, navigating to /apps/metacity/general, and turning on "compositing_manager". As to why it's not on by default, I don't know: that's a question for the packagers.
You can turn it on with
gconftool -s /apps/metacity/ general/ compositing_ manager true --type=bool
or by running gconf-editor, navigating to /apps/metacity/ general, and turning on "compositing_ manager" . As to why it's not on by default, I don't know: that's a question for the packagers.