Aperture does many things that iPhoto can't. Multiple libraries is one. You can sort of do it in iPhoto, but it requires a lot of work moving & renaming files. Aperture is a much more professional interface than iPhoto. Aperture supports Vaults, which are independent copies of your library. You can have one or more vaults, and they don't all need to be on line all the time, Aperture won't come to a screeching halt if it can't find one, it'll just update it next time it is available. Aperture supports both managed (all your images plus metadata and changes go into the library) and referenced (the library only holds the metadata & changes.) libraries. Note that in no case are your original images altered.
Since the last revision or so of Aperture and iPhoto, they can both open each other's libraries. There are some limitations when opening an Aperture-created library in iPhoto, but nothing major. All changes will be seen in both.
One reason I've found to keep both around is that the Apple online printing options are not the same for both. I've given my family custom calendars of my photos the past two years, and Aperture won't do Calendars! Yes, I know other printing houses do calendars, but I've not seen any that do the same kind that Apple does.
For $80, you can't go wrong with Aperture.