Eliezer uses the word 'vulnerability'. I think this is close to what they are trying to signal, which is 'harmless'. It is a good strategy to have a very disciplined dress code, and build a brand as having a 'dorky', squeaky-clean manner, so that people feel comfortable allowing the missionaries in their home. In my home town anyway, they went door to door and I had no qualms about inviting them in, knowing that any odd behavior would be newsworthy, and quickly become widely known, exactly because the branding is so strong.