When is bypass surgery helpful?

I'm a 67 year old male with excellent dietary and exercise habits. Nevertheless, I have considerable buildup of plaque in several coronary arteries and I have had two heart attacks. The most recent was about 10 days ago. I received two stents after the attack. I currently take a beta-blocker, an ace inhibitor, aspirin, and Plavix. I asked my cardiologist whether bypass surgery would make sense in my case and he thought it would not. I didn't really understand his reasons (other than the risk of any serious surgery) and was too weak to discuss it at the time. Can anyone help?