Shure, a mockup scene helps to visualize game in development.
So if Art is already there (eg these content packs), it
can help to determine the style direction of the game.

But this mockup should be kept seperate (just a stage) from the actual game-code
until the game-architecture has matured enough to implement
the art-assets propperly.

(Hobby)Developers should not build a game around a Gun or Soldiermodel.