Yo, includes are the bomb-diggity. Here's a screenie of my main layout's event sheet:
It has no events in it! It just has includes to other event sheets. Modularity is a beautiful thing.
By the time I'm done with the main engine I estimate that each layout will probably only have between 1 and 10 unique events, just to control level-specific stuff. Everything else will be pre-made and simply dropped into place.