Missions terminals are not the primary way to get content, and I have never seen them as doing that in sandbox titles, they are an adjunct to the sandbox. The sandbox itself has no real direction, we chose what we want to do within the available framework, perhaps with friends, and typically for longer periods of play. By its nature, a sandbox, unlike directed missions is unpredictable.
However, I do personally feel that directed content such as that provided by mission terminals is essential to the success of any sandbox. They deliver content in bite sized chunks, often focusing on specific areas of activity, they also provide a structure and a means to grind towards specific goals or rewards. They allow the player access to the mechanics and content they have most interest in, even when they have limited free time. Depending on the game they can also provide a focal point that may otherwise not exist or be hard to find in huge open play scapes.
Some of the best friends I have made in gaming have been as a result of grouping with randoms to grind missions. Done correctly, mission dispensers, whatever their guise, can add great depth and re-playability to a game.
