[Marcello]

"Depends on the defence you have in mind. I agree that, as Sniper23 said, it would be pointless to bury Abramses and such, but all users of modern material aren't masters of mobile warfare tactics. I'm thinking, why not, in 20 or 50 years, see Abrams turret dotted along ridgelines on potential border flashpoints of, say, Egypt, Quwait, Saudi Arabia..."

The game deals with stuff up until to 2020. No way their Abrams would be released for such use by then.
The problem I have with this is that for a tank what you are looking for is already covered by the entrenching option, which models the whole affair much more realistically than the class 0 does, such as the tank being able to leave the position etc..

"If you consider a really, really fixed defense (I agree, again, that there's little point to it in modern warfare), even modern material can be worth a decent preformed concrete wall and overhead cover."

You can't give overhead cover to a turret without greatly diminishing its arc of fire.