I take dozerblades because, why not? Why wouldn't you go through terrain with that thing? You could end up taking a turn or two longer getting where you want to go by taking a path that leads around terrain. I suppose you could skip buying it and just go through terrain anyway, banking on the idea that you won't roll a 1 those few times, but it has and does happen. For a mere 5 points it's almost a waste NOT to get it. Besides, you're already spending a ton of points on the vehicle and the dudes inside it, why help your opponent render it useless by throwing in the chance for it to be immobilized?
Maybe you play differently than I but dozerblades function the same through any terrain: buildings, trees, whatever. We always play it at my LGS that vehicles, troops, anything, can go straight through any terrain as long as it makes the proper rolls.
I still don't see a point in taking a storm bolter though. I'm not trying to start a fight here but if you're worried about compounding weapon destroyed results eventually defaulting to an immobilized chances are your opponent is going to roll something better anyway (or different) and not even make a different how many guns are still on it. I still see the 5 pts you'd put in for a storm bolter has much less value in the long run than dozer blades.