It's clearly more invasive to lengthen the ramus but when it's really short, as in anterior open bite cases, seems that the extra invasiveness would be worthwhile. Plus, it avoids excessive advancement of mandible, which itself carries it's own risks to IAN and TMJ, not to mention aesthetics. At the end of the day, it's only a few mm however so I can see why many practitioners don't want to bother. Surgery mills.