I've got several, Leeniek, and use them often.
You're right about what your research has uncovered; the fancy, decorated ones are used primarily for serving.
That aside, there are three types of cooking tagines:
1. Those made of modern materials, such as enamaled iron. Personally I see no point to them, as they're really no different than any other braising container.
2. Glazed tagines. These bring a modicum of aesthetics to the table, but there are some drawbacks. The biggest of these is that unless you use a heat diffuser you cannot use them on top of the stove. The glaze will craze and crack, and the clay, itself, might crack because of the heat differential when put on a hot point.
3. Unglazed tagines. These are the most traditional, and the most efficient. They can be used either over direct heat (such as your cooktop, or the traditional charcoal oven), or in the oven. And they take on a patina, over time, that is both pretty and which contributes to the flavor of the tagine (the food, not the container).
What makes tagines special is the mechanics of how they work. As the liquid in the dish evaporates it condenses on the conical cover, and, in effect, rains back down on the food. This keeps it moist and flavorsome.
Hope this helps.