I think they both served their own purpose, and within the show when they switched out for Natalie they brought it to light why they were both different and at the same time helpful. Sharona was Monk's nurse. She was clinical and sometimes a bit cold, but what she tried to do was help Monk overcome his issues. This includes giving him the tough love treatment and attempting to get him to do things for himself.

Natalie on the other hand comes as someone who was helped by him and volunteers to be his assistant. While she also has to deal with his issues, she is not coming from the medical standpoint where he's a patient and needs to overcome his problems. What was more important to her was that Monk needed to help other people, like he did her. She was there for the people who could benefit from Monk's genius, while Sharona was there for Monk to overcome his issues, which sometimes included helping others.