Smoothness is attained by learning to pedal efficiently. It reduces wasted energy and enhances your ability to engage fast twitch muscles. If you have ever seen a strong, undisciplined rider schooled by a less strong but trained and experienced rider, you know what I mean. There are other factors involved of course. When I first started cycling, about 40 years ago, I bought a book by Greg LeMond. In it he said, "If you can't afford a coach, buy rollers." I took that to heart and have never regretted it. I also have a childhood friend who was a cyclist. We met when we were 7 and went through school together up through our first year of college. We talk all the time. Back in the day he was always trying to get me on a bike but I was a football/baseball guy. Ended up going through college on a football scholarship. But, I digress. When he was 15 he was the youngest ever National Jr. Champion in his cycling event. He also went on to be in the '64 Olympics in Tokyo. A serious crash ended his career. In any event, he is constantly telling me that leg speed is the foundation of cycling and that rollers did that for him. So, I am going to stand by what I have experienced to be true. Perhaps we will just have to agree to disagree.