GGJ,
This is a common reply sternly given to many people who encounter this problem, (its actually quite common in the apple community as a whole as a response to other problems/quirks of the system) that is, to say there is no problem.
This is incorrect. Many people, myself included have encountered a problem wherein a particular program or group of programs will hold large amounts of RAM inactive. This symptom is not alleviated by anything besides a memory purge or a complete restart. Closing all programs, waiting, relaunching finder, etc don't help. The inactive memory stays very low and the computer's processing slows to a halt. Opening or operating any other program (especially memory hungry ones) is slow or impossible.
I (and others) have encountered this problem most often with virtual machines such as parallels.
In short, immediately after restarting or purging memory, the system functions well again. Opening and running any program is quick and smooth, and inactive memory is confirmed to be low. The previous state of inactive memory being several gigs (and not decreasing over any length of time regardless of programs) and after purge/restart being back to normal (only a few MB an fluctuating with programs & time) may not be the causative diagnostic, but it undeniably is a symptom, however unrelated to the actual root cause. If the observation is highly correlated with the problem it is a useful symptom.
You may not have experienced this problem before, but it is frustrating and vast amounts of personal searching has yielded three answers, only two of which actually fix the problem. These are memory purge and restarting. The answer that does not help takes many forms: "there is no a problem," "you don't understand how OSX RAM works," or "It'll fix itself."
I see your comments and have been directed to your link before, but the bottom line is it doesn't fix the problem, (which contrary to your assertions) are very real. I understand that OSX is good at handling memory, but there is clearly a glitch that is present that is only remedied as stated above.
In the spirit of an analogy. The alternator on my car fails. The tow-truck guy (you) doesn't perform any diagnosis but tells me there is no problem, replacing the alternator wont help because in a normally running car the alternator works fine. I just don't understand how alternators work. You then drive off. My problem is not resolved and I am only left more frustrated.
Not the best analogy and I apologize for the long post but I hope you may be able to see where I and others are coming from. By all means if you have an actual solution to this besides "there is no problem" I (and others) would love you for it.
For clarification I have a mid 2010 macbook pro with the problem persistant from snow-leopard and Lion.
http://www.electrictoolbox.com/purge-free-inactive-memory-mac-osx/