I'm not debating about what's actually in use. My main system has 8 GB, and I would be willing to bet during my average session, I don't touch half of that. But sometimes I do use quite a bit, and I wouldn't want to run into an issue at that point. I have games that will consume 2+ GB of memory. It isn't tough to do with video editing software or Adobe Design software, either. My wife has already shown me Facebook games with Flash memory leaks consuming 1 GB of memory. Damn Farmville.
You may not use it up all the time, but wouldn't you want to know it is there, accessible and available, should you need it? My 325xi only has 190 hp, so it might be a bad example here, but I'm sure I don't use all that driving around town. But when I need to accelerate on a highway, or pass a truck or a swerving driver, I'm very glad to be able to use it all.