I agree that dragging velocity in the controller pane is horrible, I don't know why that has been allowed to persist for all this time. I should imagine that enough people have complained about it by now for it to be a known issue.
In the meantime though, velocity dragging in the piano roll is a perfectly adequate workaround - in fact I much prefer to edit velocities here because you're always 100% sure of which note you're editing - with a busy part, it's sometimes hard to see which velocity bar is tied to which note when you're working in the controller pane.