I understand now why you're saying that assigning a variable in the layout makes it available in sub-modules.
From what you're saying you're doing this in an overall layout (webapps or shop). Indeed, there you would get that effect. Overalls are rendered a bit more "special" than regular pages and this is (at least for now) a side-effect (I would not rely on this because of that).
If you will try this in a regular page and a module in that page you will see that this will not work (do assignment in the page and then try to use it in the module layout).
Using the mechanism to pass parameters to the modules is the way to go.
-Lucian
