The mainsheet is your elevator. Driving all over is slow. Constantly having to sheet in/out more than a foot or two means drop traveler a bit and try again.
In conditions with constant wind direction and speed? Yeah. In the real world when your trying to maintain VMG, you're doing both. Steering and sheeting constantly. Now its very small changes in direction, but its still happening. Heck, you're constantly steering just to maintain a direction, either wind angle has changed slightly, speed has changed because of wave angles, wind speed has changed, changing your apparent wind. Like I said little changes, not necessarily "driving all over".
There's a lot going on that needs constant little trims.
