I think plugins, because of the additional variables they add to the game, do take some extra memory. Additional graphics can also end up loaded when they're not actually needed.
Leaving only the necessary files also makes the zip a lot lighter and therefore faster to download.
On top of that, you kind of end up leaking the contents of some paid projects by forgetting to take their graphics out of your other projects. By that, I mean there's Geeks VS Superhero CG in this game's files, for example.