Not really true games can and have supported 3D starmaps and indeed some have it's not an issue of computational power or even memory although the 3rd co-ordinate will have a slight effect.

It's more a question of the utility of such a map for stratergy given that you still have to show a 2D projection of it on your 2D monitor screen. Which limits the utility of the 3rd dimension making it not that useful since to get all the information you really need about star proximity and actual position you need to rotate the map to get different 2D projections.

Games that have used a 3D starmap generally seperate the star map from the objects orbiting the starts having a strategic in 3D and a representation of each solar system seperatly. That's to reduce visual complexity and because of scale issues if you want your map to be at all realistic.

Just to complicate matters GC3 actually currently has a 2D projection of a 3D representation of a 2D map since the objects displayed are 3D.

I think we can stop beating a dead horse now.

Until they publish some new information for us to discuss beating dead horses is all we can do here. :'(

  There is no such thing as a 3D map.

What you could have is a 3D Map projected onto a 2D Surface (your monitor).

Which adds all sorts of needless complications.  Since the viewing surface is 2D, it simplifies things greatly for the map to also be 2D.

When monitors become 3D holographic images hovering over your desk, then a 3D map will make sense.

You can't be serious, right? In that case we don't see 3D, we see 2D in a not-so-flat-and-wet surface that is interpreted as 3D by the brain, and I'm not referring to a monitor. 

Not to be pedantic but that is the truth. your eyes see in two dimensions meassuring the relative intensity of light at the surface of the retina. Your brain takes the input from your eyes and does all sorts of clever visual processing of the two images using steroscopy and various contextual tricks to undestand what it sees in 3 dimensions.

Traditional 3D game enignes render a 2D projection of a 3D space, that is they model the game world in 3D but have to calcualte a 2D projection to show on the 2D screen. They use tricks like perspective which helps your brain understand the image in a 3D context. However it does not include steroscopic information for example so you still see it to a degree as 2D. Actually what might be interesting is to see if children who grew up playing FPS games froma young age don't use steroscopic information to judge distance in real life because their brain developed using just perspective and parallax movement to judge distance and 3D spacial relations.

Anyway as I say in GC3 the map is actually 2D but represented in 3D (which is why you can rotate and zoom to a degree), then rendered  to a 2D projection.

That aside the question here was should the map itself be 3D to mirror the arrangement of stars in space (not space time which is 4 dimensional), the pros are it's more realistic (however unless scales where also more realistic what is gained here)? The downside to see how close some objects actually where you would have to continually rotate the map, also to look for objects that had become hidden behind objects.

Personally I found the explaination of the out of scale 2D map given in character in GC2 to be amusing and somewhat credable and see nothing to gain from the 3D map which I have seen before in games like ascendency.

  Another curious fact about galaxies in general, and the milky way in particular:

The Milky way has a diameter of between 100-120 thousand light-years.  It has a thickness of about 1000 light-years.

So a map of our galaxy that was 100 by 100 would be flat.  In that the time to move from one square to another would be the same to traverse the entire thickness.