I love my packing cubes! When we travel, I always spread each person's clothes out among all our suitcases, in case of lost luggage. We all have different-colored packing cubes, so it's a snap to reorganize our wardrobes once we arrive.
Another reason I love cubes...in a tight space such as a cruise cabin, I stack them and use them like bureau drawers to keep my clothes organized. My swimming things go in one, shorts in another, underwear in a third, so I can find what I want quickly.
Yet another reason I love them is that I think it cuts down on the chance of something falling out of your suitcase if it gets opened by TSA. It's easier for them to search the suitcase, and if you're really worried, you can throw a business card in each bag in case it does get list.