I've noticed that in my runs, I never use the BFG unless I'm fighting the cyberdemon (and even then, I'm usually not playing rapidfire so I might use the LS or a shotgun). In my most recent Nightmare! shotgun game, I decided that maybe my shotguns weren't going to be enough for this level and that I had cells to spare, so I might as well fire off a couple of hundred cells and just take out the cyberdemon with my shotgun.
So I go up to the nearest 10 Revenant + 2 Vile pack (I had Iro 3, TaN 3 and red armour) and start shooting. I realise about 2 BFG shots in that I had accomplished little other than getting them all to 30% HP and destroying my reloading cover. With no phase devices, I died shortly thereafter.
It was a brutal reminder (I was on level 24) of why I never use the BFG, because it MIGHT kill everything but it DEFINITELY destroys all of your cover.
I suggest making the chance for a BFG to destroy walls MUCH lower, but keep the item destruction rate about the same as it is now.
Or change it so that the BFG has 2 explosion radiuses, one for full enemy damage and high wall destruction chance, one for half enemy damage and no wall destruction chance. The first explosion would be as large as a normal rocket, the second would be anything that you can see, no matter how far away it is.