Is free ram really wasted?

Is free ram really wasted?

I'd be grateful if you can give me some clarifications about memory
management in Android. I know, so many times has been told that task
killers are useless, Android is not Windows, it is Linux based so it has a
different ram management where we can say that free ram is wasted ram. But
about this "wasted" word I'd have some questions. Actually my device (Acer
Liquid Metal running cyanogenmod 7) has 120MB free ram. Now if I give to
the terminal this command: cat /proc/meminfo I discover that I have almost
100MB as cache and 20MB as MemFree, and I noticed that If I have more free
ram I have more cache used by OS to speed things up. At this point I'm
asking to myself: Is it really wasted or unused the free ram? In my case
the really wasted ram is only 20MB, but 100MB are used as cache where it
grows when I have more free ram so it shouldn't be wasted... Am I wrong? I
can't deny that this doubt is started after I read these two links: here
and here. I have another question: if it's really wasted, why my device
becomes laggy when the ram is below 40-50MB? I don't use any task killers
because I know they are useful, I just changed the minfree values through
the v6 supercharger script. I hope you can help me to clarify these my
doubts. Thank you very much in advance :)