This question brought a recent visitor to this blog: Does a medical statement from my doctor help with social security disability?
The short answer is YES! A medical source statement from your treating physician regarding your functional limitations is usually the most powerful evidence available for a Social Security disability claim.
It is not just common sense that your treating doctor knows you best. Social Security regulations (see 20 C.F.R. 404.1527(d)(2) and Social Security Ruling 96-2p) state that your doctor's opinion, in ordinary circumstances, is more important than the opinions of other doctors who may have examined you just once, or not at all.
A medical opinion from the treating doctor is so important that when I receive one, I write a handwritten note to thank the doctor for taking the time to complete the medical source statement. I actually have special stationery for this. I want to let the doctor know just how important the opinion is for the patient's disability claim, and how much the effort is appreciated. I do this because medical source statements win claims.