I think the answer is the collision , Map Entities are basically Bsp based blocks that are treated as models , the advantage of using them is you can assign actions to them , while still mainting their accurate collision , which is why they are used for things like elevators and the like. This were implemented before gamestudio had correct polygon collission , no need to really use them if your building your levels out of models with polygon collision for everything. On the other hand , if your using regular blocks for your levels , and you want something like an elevator to carry the character or a door to block a path , then using a map entity is the solution , because you cant assing an action to a block , and models have poor collision. Hope this clears it up a bit for you.