Chas-m, one thing that has been a bit of an eye opener using Free Memory Pro is how well my MBP (with it's stock standard 4 Gb RAM running Yosemite) runs with only say 150 Mb free. So I suppose the value of this exercise has been becoming more aware of what uses what and how little free memory is required. I might be hovering on the limit but so long as I don't reach it everything works fine.
The reason is because "free" RAM (as reported by the system) is only part of your available RAM. As innumerable articles on Activity Monitor will tell you, "inactive" RAM is also "free," but Apple classifies it differently because it contains previous RAM contents no longer in use that you might reactivate by opening the app again, so as to perform better.
So in fact, only "wired" (RAM reserved for essential system functions) and "active" (RAM being used by the open fonts, applications, menubar items, widgets, etc) are actually "in use" and everything else is readily available when called for.
At the moment for example, I have 1.4GB of "wired" RAM, and 9.34GB of in-use RAM. This is out of 16GB of RAM. But "free" tells me I have only about 1GB of RAM "free" (I'm using the terms used by pre-Yosemite Activity Monitor. Yosemite has a different vocabulary) when in fact I have 6GB of available, not-being-used RAM.
In Yosemite, RAM is divided into App Memory ("active"), File Cache ("inactive"), Wired and "Compressed" (for me this is ALWAYS zero). Yosemite makes use of RAM compression for users with less RAM (say 8GB or even 4GB) in order to make programs run as smoothly as possible. That's why people who have 8GB of RAM but don't run more than say two programs at a time are likely not seeing any RAM issues, just like I don't with 16GB of RAM. Ooh you clever Apple engineers!
So "memory cleaning" apps just run the PURGE command on demand, which just dumps the "inactive" RAM into "free" RAM and makes not even the tiniest whit of difference in performance, and in fact can interfere with memory compression that would have actually worked better.