The difference for the 234/4 is that it had no turret and a bigger gun. Is that different enough for you? 
All in all I would agree with your statement (which is why I think a Pershing, for example, would be redundant since we have the Tiger), but I personally think there is enough dis-similarity between the Greyhound and the German ACs to merit their inclusion.
I think a point is missed when folks talk about an item not adding anything new to the game.
There are quite a few items that add nothing significantly different to the game.
Using GVs as an example. From a gameplay standpoint there isnt much different between the Sherman and the Panzer other then the Shermans smoke shells dont shoot as far. I've gotten kills and been killed just as easily in a sherman as I have in a panzer and vise verse.
Yea the sherman is faster moving. But you dont see too many battles of maneuver in the ground game here LOL
They dont always have to add much different to the game
What they do do is add a national representation to the game.
So you can actually have Tigers against Pershings, M8s against Panzers and SdKfz 234 against Shermans
This isnt good just for scenareos. But for the MA as well as it gives people more options then just a one country representation ride.
And for the record. While its preferred and its easier. You dont have to be at close range to do damage with an M8.
Im sure quite a few in game can testify how much of a pain in the arse I can be in an M8 LOL