Are we sure it's a bad thing when RAM usage goes up?

[kirstein]

I'm reading lots of people trying to figure out how to keep RAM usage down. What I'm wondering is if that's really productive. Seems to me that as long as the OS properly manages it, it should be OK if RAM usage get into the 90% range.

If the OS is using RAM to cache commonly used chunks of code, and if it has a decent routine for freeing up RAM as needed, what's the problem. It could actually slow things down to keep purging RAM.

I'll admit I don't know much about what's going on in the kernel or exactly why RAM usage grows over time. Is there anyone who has dug deep enough into the system to know for sure what's going on in there?