At the moment, I am finding being an alien very hard, especially when I climb on walls. Very often, the walls and the ceiling are not where near as flat as the floor, and you get very easily confused where you are. With 3rd person, you would be able to see where you are, and not have the camera shaking as much.
I don't think it will remove any of the capabilities of aliens since most do not do not have any shooting function. For the ones who do have such a function, I guess we could make a type of pointer where the thing they shoot is going to land. Of course, to avoid having a perfect aim, the pointer will indicate the area where it is going to land, instead of the exact spot.
Of course, humans would not be able to have this 3rd person mode since it will be hard to them to shoot.