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I disagree that unused memory is wasted memory. The system, and programs themselves, need room to do their work. The more stuff it can keep in memory, the better. Memory filled just for the sake of filling it is wasted since it takes working space out.
You're kind of contradicting yourself here. You're saying that it's better to keep more stuff in memory but then also saying that having unused memory is good too.
The way the OS manages memory is a balancing act. It needs to divide up the physical memory between the programs you're running, various caches, and free space such that whatever you do with your computer, performance will be good and the time spent swapping data to/from the disk is minimized.
This is tricky. The OS has to essentially guess how much memory each program might want to use in the near future, and decide how much of each program's memory should be moved to the disk (if any) since it's not likely to be used in the near future (or ever). It also has to "guess" how much memory is a good amount to keep free in case you want to run a new program. It also has to decide how much memory to devote to the file/disk cache vs. the cache for memory compression.
So when you look at the all the numbers in the Memory tab of Activity Monitor, you're really just seeing a bunch of the OS's best guesses about how to allocate memory so that everything runs smoothly.
Looking at these numbers is probably pointless to anybody who doesn't work at Apple. The only useful bit is the memory pressure graph, which seems to essentially give the OS's "opinion" about whether or not you would benefit from having more memory. (Yellow or red = yes.)
