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
Which would you take though - smoothness or power? I can always ride smoothly (esp on the rollers) if I'm not going at a pace that's very hard for me. Crank it up to VO2 or above levels though, and it gets substantially more difficult.
To me at least, it seems that the smoothness is more a function of riding within your O2 limits and less a function of technique. I don't think all the technique in the world would allow me to ride smoothly if I'm going all-out on the rollers.
Trouble is, for me, the key benefit of the roller/trainer is to be able to go hard. No traffic, no stops, and good reliable computer data. Backing off the top speed just so I can go smoothly seems to detract from this training stimulus.