Please define "keep light". Having tons of multisite files shipping with core while you use it for a single blog is far away from "being a light core".

I think what really would help is a build system that is able create packages for the different usage profiles and needs. That build system could bundle core plugins as well - just to show the picture.

Eg. Classic could ship with hello dolly and akismet for that Matt can sleep at night, there could be multisite package and one package for single site use. Can be something valuable for the current wordpress.org redesign as well. Just to add some ideas.