If you run an application that needs the KDE (or GNOME) libs and you focus on it (e.g. an IDE, a browser, a document viewer, etc) then it doesn't matter that it

If you run an application that needs the KDE (or GNOME) libs and you focus on it (e.g. an IDE, a browser, a document viewer, etc) then it doesn't matter that it uses the KDE/GNOME libs since you can consider them as part of the program itself and they'll be gone once you close the program (still take disk space but that isn't much of an issue outside of VMs). Auxiliary stuff you leave at the background (e.g. an IM or music player) tend to have multiple applications and you can always find an alternative. There are exceptions of course but they can be limited in general. And chances are you want to avoid the latter stuff for the former stuff, it isn't like RAM exists to be unused (even for file caching). In other words, if i run a heavy program as a primary task then there is even more of a reason to want auxiliary programs to be lightweight so that this program will have more resources for itself. (Also as an extra bonus, sometimes as a programmer i want the primary programs - IDE - to be lightweight themselves since i want the resources to be available for the program i am working on)