Normals are widley used in games to reduce the enviroment poly count and the speed that it can be calculated. Some gaming engines crank out obsene framerates, and using normals mapping is just one of many things used to cut down on the calculations to achieve those framerates. Even thou it is one of the fastests (frame rate) way to do it, not all game engines use this technique.

All the methods have advantages and disadvantages, decieding on which ones to use depends on a lot of factors in the scene.

Some say a standard rule is to use displacement if the object is close to the camera. Bump works fine as long as it is not a lot of it. Use bump for things in the background that have a decent poly count. Use normals on anything that is very low poly. Yada, Yada, Yada....

Sticking to that theory limits what you can do with them, you can mix them as well to get results that are not achieveable using only one type.

Use what works for what you are trying to achieve.

 

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