First off, I think adding manual calibration would be good for the simmers and not good for the gamers. It would lengthen the learning curve quite a bit. I can see either way.
Secondly, I agree that currently it is far too easy to be accurate while level bombing. There are simply no variables the bombardier needs to worry about other than giving the bomber barely enough time to calibrate and then let loose the dogs of war. The challenge is minimal, really.
Thirdly, I've been advocating for HTC to make carpet bombing more of a norm than it currently is. Actually, I rarely see it used now. The reason carpet bombing was used in WWII was the need to saturate an area to more guarantee the target was damaged as desired. Keep in mind the pilots had to deal with winds and up drafts, there was no such thing as perfect level and straight flight like we are afforded in AH. Keep in mind that bombardiers had to content with cross winds and an ever shifting plane, too. It was no picnic and it certainly was no where as easy as it is in AH. Bombing in AH is very luxurious in comparison. On that note, I'm not suggesting HTC take away auto pilot, etc, but a cross wind or a built in gradual "spread" penalty depending on altitude would go along ways in lending to more "realism", per say.
Why not a 5mph cross wind above 10k, 8mph at 15k, and 10mph above 20k???