At the beginning of my career I toyed with the idea of specializing in game AI but decided I didn't want to write games for a living. I didn't entirely like what games were doing to my friends.
I recall that Halo 2 made a big deal out of the fact that part of compiling the game maps included putting metadata in about terrain and cover, which is why some of the mobs would hide behind crates when wounded. You had to play hide and seek with them the same way they had to do with you. I'd really like more of this. In theory these bad guys have been in this area for a long time. Of course they should use the terrain to their advantage.
And as you mentioned about lobbing grenades around corner, the reverse should also be true. If one grenade blew up in this spot nothing is stopping another one from coming. You should avoid that area for a while, and move to where you can shoot anyone who comes into it.
But I think the real problem is this Rambo model of games. In reality, a single assailant against a large number of armed defenders should never be able to attack their way into a base. They should be pinned down by the occupying forces until the ammo runs out. You could sneak in, but you can't just show up with a rifle or machine gun and get anywhere. But aside from the possible solution of harvesting comedy gold from this trope, this reality based model would be really, really boring.