How do you get better ground clearance with a narrower BB? I would argue it is the other way around (assuming we are talking cornering?)With 5 or 6 speed, you don't need the 150mm rear as the 135 can be made dishless, without the 150 rear, you don't need the 83mm BB so you get better ground clearance and Q factor. Doesn't help with room for suspension on some designs though. I don't think the slightly wider stance of 83mm BB would make much difference to stiffness or strength.
A wider BB also gives a lot more stability for DH (tripod example) and in my opinion this is very noticable.
There seems to be huge demand for less gears though, I only run 7, although not to full advantage as it is 9 speed - 2. And have never missed to extra gears.