If at all possible I ride from my house ... that being said, if it's a short trip and the car is already full (like a few miles), I use a hitch rack with the bike standing on its wheels. For longer trips, if there is room, and I'm a great packer, the bike is in the car. I've gotten fairly adept at layering our luggage in the back of our X5 so that the bike can go on top. When we traveled north from Florida this year there were three layers in the back -- luggage, bike 1, bike 2. If I cannot manage to get the bike inside, I'll use a bike box on the roof.
I hate leaving the bike(s) exposed. At 80mph sand and water are pushed into all the bearings; grease is driven out. The bike is sand-blasted. If I had one of those hitch-mount luggage racks, the luggage would be out there; the bike inside.
With 140k miles on my 2004 X5, the bike will soon be worth more than the car, but not if I leave it out in the elements. Well, that's not completely true -- I would just end up replacing Groups more often.