I disagree with the assessment. Those 4th liners will play 10-15 minutes. That's 16-25% of the whole game. That is not insignificant and shouldn't be just filler. If the Ducks want to take a step forward, the whole team has to contribute. Guys like Johnston, Harkins, Leason, and Lundestrom have not positively contributed to the overall success of the team. There are players who can do that.
You are correct; they are not the biggest problem. However, they are a problem, and Verbeek should be trying to address all the problems and not just one. If these players were replaced with PK specialists, for example, it could be a real net positive for the team.