On the contrary. Ideally you'd be using 80-90% of your RAM at all times. Ever notice that the more RAM you have, the more your computer uses? Unused RAM is wasted RAM. Recent versions of Windows for example cache things into RAM to speed up the loading of frequently accessed applications. If you run low on RAM, then it frees up room as needed.
I don't think the parent comment/thread was correctly read. The point is that an ideal OS would consume no resources leaving them entirely for the applications to use. The OS caching frequently used applications is different from the OS itself needing resources.