Sorry but there was something ridiculous about citizens of foreign countries bowing to a box. The US fought a war to rid themselves of the British monarchy; other countries, while they may not have made quite such a spectacular statement, also chose not to have the British Monarch as their head of state. IMO it's rude to require citizens of countries who have no allegiance to the British crown to bow or curtsey to its representatives.
IMO the Duke of Kent was correct and, after all, it was to him and his family that people were being asked to bow/curtsey. If he didn't mind then why should anyone else? As we have discussed in the protocol thread, the Royal Family's website says that bowing and curtseying are optional when meeting any member of the Royal Family (the Queen included).
The Duke of Kent, his wife and his mother have been longstanding and loyal patrons of Wimbledon. I don't think he should give up his position until he wants to - and hopefully not for many more years to come.