I think that mortgage brokers begin to provide a lot or value to those who deviate form the traditional high documented income high down payment low DTI.
If I am a sergeant on the Police force with a wife who is a nurse and household income is 200K and have 200K in cash and are looking for a 500K home. I think that it is pretty safe to say that that person can pretty easily shop around.
On the other hand if I am self employed and wife is a salesperson and we make the same amount of money but document it differently then I think the added complexity introduces opportunities where broker expertise comes into play.