Its all about multitasking and what you multitask. i.e. watching a movie on a second monitor while running a minimized virtual machine on another, while photoshop is open, a web browser is open, word is open, a game is minimised... you get the idea.
I'm one of those people who hates to close and open applications when I'm using them constantly, and typically system uptime of a week/two at a time, I can't be bothered to load up apps! from reboot

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If your memory fills up, the OS dumps the stuff that's minimized/unused onto the hard disk. here is the bottleneck, your Hard Disk! to maximise, OS need to free up RAM (again ...) needs to dump what it has loaded and load memory for what you maximise and set focus to.
This is slow. So most apps use 4GB ram, there just is no need for more unless your rending massive print work on illustrator or running memory intensive applications like matlab or mathematica, or your a heavy multitasker.
edit: oh and more relevant, I would opt for larger memory sticks if I'm planning to upgrade, would hate to throw stuff away ... not because it would be faster ...