I do happen to have an 8 GB machine, and almost never reboot (only reboot for OS updates).
Here's what I don't understand - why would anyone in their right mind purchase a $2000 laptop and then not spend the 20 minutes and 100 bucks to max out the memory on the thing? It's the easiest thing in the world to do, and basically means you never have to worry about memory usage again.
Recall that what "matters" is really that second line, which translates to 740MB being really "used" and 3.1GB being just "stuff I happened to read from disk at one point", which as it happens includes rather a lot of media files. Loading another 4GB of media into RAM isn't going to help my system performance any.
This is with a respectable Linux dev loadout, but I'm not running my VMs, but that still tends not to strain my system any. $100 on RAM would just be a wasted $100.
And-or get an SSD - the heavyweight disk-space users like movies and music are trivial to move to an external drive on a Mac, and with an SSD so much disk-thrashing pain just goes away. It's pretty great.
After noting my rather older MBP with 8GB RAM was outperforming my new iMac, I spent $16 for 4GB of RAM (doubling the total to 8GB); the iMac's performance improved immeasurably. By far the best bang for the buck on OSX is maxing out the RAM. (Well, at least until I can afford a solid state drive of adequate size.)
Here's what I don't understand - why would anyone in their right mind purchase a $2000 laptop and then not spend the 20 minutes and 100 bucks to max out the memory on the thing? It's the easiest thing in the world to do, and basically means you never have to worry about memory usage again.