Comparing Imams to doctors is ok assuming that we believe that Imams can do more for us BECAUSE of supplications. In the month of Ramadhan we recite Dua'a Iftitah in which it says: (Discussing 12th Imam) "O Allah, send him to call the people unto Thy Book for establishing Thy religion and make him succeed in the earth, as Thou caused those who were before him to succeed and establish for him his faith which Thou has approved for him, and give him in exchange safety after fear. He serves Thee. He ascribes nothing as partner unto Thee." You also asked why do Shia not call on Imam directly, yet there are some shias who do. You might be surprised to hear that many of the Molana/Sheiks/Zakir consistently remind us that Imam Ali (as) as great as he was he still prayed to Allah s w t and tells us to not forget that. I'm also going to admit to some people in the community feel a bit more strongly about Imam Ali and have hints of Nusariyat. A true shia as stated by the 5th Imam Imam Muhammad al-Baqir (as) once said to one of his companions: "O Jabir it is not enough that a person says; I am a Shia and I love the Prophet (sa) and the Prophet's family and the Imams. By Allah, a Shia is the one who is perfectly pious and obedient to Allah's commands. Anyone else is not a Shia no matter how much they say they love Imam Ali (as) and no matter what they call themselves. O Jabir Our Shias are known by these signs: ( a ) They are truthful, trustworthy and loyal; ( b ) They always remember Allah; ( c ) They offer their prayers, observe fasts, and recite Qur'an; ( d ) They help their neighbors, take care of orphans, and say nothing but good of people; ( e ) They act nicely towards their parents; ( f ) They are worthy of peoples trust and confidence.