I use only top quality memory cards and have shot a very large amount of pictures over the years on Nikon D200, D300 and Fujifilm X100, and tend to delete poor quality pictures on spot, but have to date never encountered this problem which lead me to suspect that perhaps many of cases you've seen might be attributed to poor quality memory cards rather than camera firmware, alternatively that I use cameras with better than average firmware. Another possibility could of course also be that in your line of work you encounter these issues on a frequent basis making it seem more prevalent than it really is, after all it probably isn't a common occurrence that you have customers walking in saying "I always delete pics in camera and have never had to do recovery".
I'm not attempting to dispute that you do see this problem frequently, but am genuinely interested in your feedback as your posts made me slightly concerned abut my in camera deleting habit.