Apples and oranges. There is a lot of overlap today, especially with medical psychologists (degree in pyschopharmocology) found in most of the 1st world, and now gaining ground in the USA. However, I'd say that a psychiatrist is much better suited for organic brain disorders such as schizophrenia, while a psychologists are better suited for non-organic illnesses such as the aforementioned PTSD.
Psychology is a major people go into to get burger flipper jobs. You cant get anywhere serious in that field with just a bachelors degree in psychology.
I fail to see the relevance. We're discussing psychology as a profession, which pretty much means we're talking about Phd's. Possibly masters prepared LLC's. There are plenty of professions that require post graduate degrees or professional degrees. I don't see how the value of a bachelors applies to this discussion at all.
It is relevant.
With a 4 year degree in many other fields you can get an entry level job in that particular field. Engineers, journalists, teachers, business degrees, can all get you a spot in a real world job in your field. Not so much so psychology. The 4-year degree is just a starting point usually.
Of course any field if you want to do research or teach you go for advanced degrees.
The other interesting thing is that so many psychologists themselves are seeing a psychologist for their own issues. Another unusual aspect of this field.