Now you can't save a few structures as a module. The only way is to place them into a separate stripe. But you don't want to have excessive number of stripes on your page not to make your email weight more. Quite often there are cases when different structures belong to each other. For example, when I want to imitate a forum post with an avatar + author name (structure 1 with 2 columns), post text (structure 2 with 1 column) and a section below it with date, number of comments with icon, and reply button with icon (5 columns). In this case it would be convenient to be able to select all three structures and save them as one module. Plus possibility of moving/copying/deleting for multiple structures would be very convenient (I probably forgot about some other manipulations).

As an additional idea: When you select objects you always see their controls in the right/left menu. But for such a group of objects this menu will be empty (unless you decide to add some common properties there like paddings or background colors etc.). But even if you keep some common structure properties (which is very useful as well) there should be more free space. You can add some commands to this menu like "Group/Ungroup" for example, so that you can just select them together with one click next time.