I understand your point. You can cycle in the scrap yard when you are two line team and those two lines play most of the game and provide the offense, which is relied on every night to win games: Good: Pittsburgh, Bad: Edmonton. When you are 4 line team, you need stability on that 4th line as you are relying on them to contribute more to the overall game.
The Isles are not built like a 2 line team, well, because they do not have that elite talent up front to tip the roster in that favor. So, supporting the 4 line team with actual NHL players is what the Isles are doing. Rolling out the scrap yard, adds risk to the game since scoring and defense are distributed. Very easy to think the scrap yard can just play, they are adequate, cheaper, etc. but time and time again, teams (coaches) want consistent NHL players. We know fans think these things are interchangeable, but they are not.