I don't think the nebula effect was removed because the AI couldn't exploit it, but rather because it wasn't fun.
In theory, using the battlefield to your advantage to beat an opponent is a good thing, it's fun, even if it means another handicap to the AI. In practice, it's too random and complicated to pull it off in the campaign. The only time i managed to exploit and get a noticeable advantage from the environnement was during missions, thanks to their handcrafted environnement that suits the battle. But i never managed to do it with noticeable effect in the campaign.
The first thing that would come to mind would be "buff the environnement!" to compensate for the lackluster impact of nebulas. But buff it too much, and you end up getting a feature the player can abuse like those that Megas listed above. It would force the player to exploit it over and over to get that big impact, which would inevitably reduce the potential amount of "fun". Moreover, trying to find the proper balance for something randomly generated in the campaign would take a lot of trial and error testing.
Hence, the reason i suppose Alex dropped it. Not worth the effort to keep, nor the effort to balance it this late in the development.
I wouldn't mind seeing a modder take a shot at it though...