I can't quite see why it should be regarded as pretentious. You will find workers in many industries wearing collar and tie, look at school teachers for one. My 5 year old is taught by a male teacher who wears a tie every day. It sets a tone, weather you like it or not. I view this similarly to a PPL instructor - my 5 year old is not going to get a professional qualification from this man, but he and the system he represents don't see this as a good enough reason for him to turn up to work in jeans. What would you think about a instructor who turned up in heavy metal rock gear with died hair and piercings all over his body? I would be enlightened enough these days to see what he instructed like and how good his handling skills were, but, you may find that the owner of the school/flying club is not willing to take the risk of losing customers due to the somewhat extraordinary appearance of his instructor. Students are paying a lot of money over for this insruction so the school will want to look as professional as possible, it's all about showing a professional image and instilling confidence in this safety critical industry. Perhaps you don't need a tie to achieve that, but psychologically it helps.
PP