The title says pretty much everything about this suggestion. This would be helpful to deal with attached sub assemblies that don't depend of the main staging to be used, but are attached to decouplers, for example. If you have a big vessel with a lot of stages, it becomes pretty hard to find out where your stage that shouldn't be activated is and increases the risk of you misplacing it on the order. Once you remove them from action groups, they return to the staging column. Another nice thing would be to have command order priority for action groups, much like staging. You can decide in which order things get done by creating groups for different actions. This would give an easier implementation to the prior suggestion, since you could just set the entire staging sequence as an action group of the 'Stage' category, pretty much like landing gear and lights get automatically assigned to their respective groups in the Action Groups tab.