having an official diagnosis is only worthwhile to the extent that it opens up possibilities to help the client get his or her life together

This. Having an official diagnosis of Asperger's was basically "oh, that's nice" -- having a therapist who was ready, willing and able to help me address my social problems in terminology that made sense to me and figure out ways to cope with them was what really made a difference.

If a person has problems they need help with, "a therapist" isn't necessarily going to solve those problems -- it needs to be a therapist the person can connect with and who can help them figure out a suitable plan of action for solving them. I've run into far too many people pouring money down the therapy hole trying to deal with someone who just isn't cut out for assisting them in the way they need.