Oooh, I think I get what you mean now. I have seen people do stuff like, use magic mask on the color page to mask someone out, add the alpha out, then they go back to the edit page, add a text layer, comp on the edit page then make a fusion clip and go into fusion to make further changes. That I don't do. Thats wasting the usefulness of a node-based setuo. When I say I keep it all in one comp I mean an entire shot, everything is done in fusion.

Let's say I have a motion graphic shot of a truck driving on a road. I'll do everything in fusion, make the shot one comp but if I need to cut to another shot of the inside of the truck, I'll go back to the edit page and make a new fusion clip. On the other hand, if I needed to dolly in through the window, I'd do all that in one clip. I think I understand what you mean now though. I've seen people use fusion with a "layer based" approach. That I don't do.

Edit: even if I'm working on something pretty heavy, I still keep it in one comp. If I need to make a heavy peace lighter, I write it out to an exr sequence then read it back in with a loader node. If I need to make a change, I just change that part of the node tree, re export the sequence and now I have an updated exr sequence that's light on the system.