Honestly, this is kind of a dumb design if you think about it. A Macharius is capable of using the vulcan mega bolter on a traversable turret. So why wouldn't a freaking Baneblade chassie be able to do the same?
Maybe it is because they want to keep these things very simple, and to remind people that these things probably never were ment to be used as weapons because many have been made from old STCs which were developed to be used as tractors, construction vehicles and similar things.
I think it looks awesome, just wish there was more melta burns in other places, realistically whatever put the first tiny punctures in the side would have likely been using more units to try to do the same. Right now the random shots (not even strategically placed) just seem like a toss on, but they look good nonetheless.