Free memory is wasted RAM in linux/unix environment since their memory management is too good and they'll free the memory in an instant whenever it's needed.
I agree with your statement, but I believe Merlin switched to less aggressive memory management (basically less caching) a while back and as you can see my cache size is low and the same size that it started out with. Since the cache isn't growing and only the memory used is, I still think that there's a potential for a memory leak here.
In any case, I'll wait to reboot my router until if / when I hit 0 MB free and am forced to, just to see what ultimately happens.