The trick as I see it, is not to use referenced images but rather managed images. The real advantage, at least in my opinion, of multiple libraries is the ability to move them to different drives as the need arises. With referenced images you're not moving the library and images, just the library.
With a managed library, you cannot get duplicate images, since a given image will only reside in one library.
Fitting a 600 gb Aperture library, in my case, on an ssd is not possible. Losing the significant advantage of having that library on the ssd is out of the cards. That's why its split between current and past. The current travels with me along with its images.
I'll have been away from home 5 months this year between USA, Far East and European travel. A managed library would be a colossal pita. For others, managed works fine as long as they don't mind working off externals.
In my case its also a matter of priorities. Spending money on, and hauling around, computer hardware just doesn't compete with travel, photography, good food, and a long list of other things I enjoy doing.