I think it's a good idea to keep indoor litter boxes even if your cats go outside. There may be occasions when you have to confine them indoors (eg after moving or following surgery) and it will be helpful then if they are used to using a litter box. If you keep them indoors at night (recommended) it's also a good idea to provide a litter box in case your cat needs to go and can't get outside. If your cats will actually use a litter box instead of/as well as outside it might also help to keep them close to home, not to mention making it easier for you to identify if they have a health problem and making it less likely they will use someone else's garden as a litter tray!
Mine are indoor/outdoor but I have litter boxes on each floor of the house, which are used by all cats. If it rains and they don't go out, then I notice quite a difference in the state of the boxes! But if one is sick I need to be able to get a sample easily, also they use the boxes at night.