Modeling using different objects in one applies for many game models. You may have a lo-poly model with a few objects on top of each other because trying to model the detail from one area to the next may be easier by just using another object rather than having the hassle of trying to get geometry to work. You usually end up making more faces just to get the same result in the end.
The tank video series explains this pretty well.
But yeah. For parts that are animated you may end up with a few different objects. Eyes, toungue, teeth, even arms and legs can end up being separate objects. And for robotic or non-humanoid characters that goes up even more.
Things that are modeled as one piece tend to be sculpted works that are stationary and will not be moving. Or simple geometry.
