It is a shame that people had been led to believe in and uphold this ban. It's falsity is blatantly apparant if you just spend the time to find out more and use your own mind to analyse the situation just like how Buddha Shakyamuni stressed and taught us to do. The matter of the fact is that people simply do not want to think for themselves, but rather follow what they are told to do, not just in spirituality but in all aspects of life.
But if you just paused, studied the issue and contemplated you would come to the same conclusions as Big Uncle has posted here. Take myself for example, even though I didn't know much about Dorje Shugden before being asked to read up about his practice, I was always led to believe that he was a demon, and not a fully enlightened being, as he really is.