I am a bit obsessive about being punctual to appointments and really don't like if people are habitually late. It would not only bother me but would be a deal breaker if a therapist was repeatedly late, I would perceive it as poor professionalism and lack of respect. If someone chooses a profession that involves working with set time windows, I think they should then make the best effort to keep them intact. Being busy is not a good excuse about not respecting other people's time.