Try expanding a brush or a 3D shape and take a look at the large pile of unwanted groups, compound paths and clipping masks Illustrator has generated for you. It just adds complexity to the file and makes the object appearance tricky to work with. See example ai file attached.
If you have Astute Graphic’s Vector First Aid plugin you can remove all that stuff and see just how simple and usable your vectors can be (I recommend it), Wouldn’t it be better if Illustrator didn’t generate junk in the first place?
Please make sure Illustrator only generates the barest minimum of extra groups, compound paths and clipping masks!
Note to whichever Adobe person merged my previous feature request with "Achor point smart delete"; excess points/path complexity is not the problem I’m talking about here! I use both Vectorscribe and Vector First Aid anyway.