Choose number of workspaces in a consistent way

With Metacity activated, it is possible to choose the number of workspaces by performing a right click on the workspace selector, then selecting preferences, and then selecting the desired number of columns and lines.
With Compiz activated - by default in Gutsy... - this operation has no effect, but it should! One shouldn't have to look for another way of doing things according to the window manager he is using.