There is some validity in wanting some free space in your RAM banks. If its filled to the brim it has to wipe out and free some space to make room for a non-loaded program and data.
Also let's say the OS decides it's going to go RAM crazy and pre-load a bunch of apps you never use. That really is wasted RAM space because it's space that could have gone to an app you might want to use but since the RAM is full it needs to do extra work.
Of course in concept though empty RAM space is "wasted" for all intents and purposes, but no scenario is ever so perfect that 100% of the RAM is being used for all the stuff you need the performance for. It definitely doesn't hurt to leave some of it empty. This is especially easy on our phones since they have so much RAM capacity. I run around 900MB-1.2GB used up space, so like 800MB-500MB free RAM space left.