Those^ units are too noisy to hide under fog anyway. I mean, as soon as a kirov is built it'll let everyone know that it's "REPORTING!", then make a constant deep hollow drum sound wherever it goes. Siege choppers' propellers are louder than rocketeers' jet packs. Tanya will tell your enemy that she's ready to "ROCK 'N' ROLL!"(YR) or just literally LOL (RA2). Oh and superweapon structures can't be hidden from your enemy by default.
But yeah for the most part, fog of war would be useful. But I think it'll force people to change their strategies a lot (which isn't necessarily a bad thing). Like for example, planes will be built more often because hit n runs will be a lot easier to execute (the same goes for those fast ifvs), stopping your war miners from getting droned will almost be impossible, you'll have to repeatedly scout/anti-scout, more base defences will be built (including scattered dogs to defend against spies), the Libyan demolition truck will become useful... and so on.
It's probably possible to mod the game to have a re-shroud option (but only with the black shroud, not fog) by constantly triggering whatever is triggered when your Spy Satellite Uplink is destroyed.