It's not just about line holding, it's about smoothness in general. Maybe not important in a club ride paceline but it is for racing. It's also the kind of thing that may benefit others more than you.
See discussion starting here
http://www.bikeforums.net/showthread...1#post10464491
That 556 dude is soooo smooth.
Usually those that don't find a benefit from a form building thing either mash their way through the benefits (I have a friend that has a bad pedal stroke and rides rollers with that unsmooth pedal stroke. I dunno how, he just does) or they don't need them (Powercranks).
But having said that, I find it beneficial to ride rollers on a semi-regular basis.
As OP pointed out, not for power. It's for smoothness. You want power, trainer or outside. Smoothness, rollers, in the drops, fast and smooth.
cdr