The episode with that warp "wave" could have been a trial of a new technology meant to resolve this issue. From what I remember, that episode did not have a specific reason other that "traveling at warp via an external energy wave is cool" idea.
Hmmm, I consider the Alexander-Worf story in that ep the primary reason for the ep being there. Even though probably more screen time is given to the 'soliton' plot, to me it ultimately felt like an excuse to get Alexander in danger after disobeying his father .... and then
the "traveling at warp via an external energy wave is cool" idea applied to it.