And of course that's how you move the bike from side to side on rollers, so I think they do help with cornering skills somewhat. Riding with a group, you see some riders whose upper bodies don't move and whose front wheels don't move from side to side as they pedal, in or out of the saddle, and those guys probably ride rollers. Pedaling smoothness reflects in bike motion, or the lack of it. Pedaling smoothness probably doesn't relate to higher power output, but it does relate to endurance. Ride with folks who ride ultras and you'll see some very smooth pedaling. It's just a whole 'nother world compared to trainer riding.

I always emphasize the importance of getting rollers with resistance so that you get smooth when going hard, not just spinning easy, and can do intervals at lower cadences. In winter, I get about half my cycling hours on rollers. It makes a difference.