Eh that's a pretty weak reasoning as the Lost Children Arc is a huge one for Guts as a character. You say it reintroduces the Black Swordsman but we already knew who that was and what he stood for. What the arc was really about was reintroducing Guts to his humanity through some of his darkest moments yet. He is forced to reflect on himself for once in his mad dark quest for revenge thanks to Puck, Roshine and the events of that arc and we actually see him rising back from the morass of hate, violence and so on he's dived so deep into.
Edit: The Roshine battle was also probably one of the best in the series and shows just how far Guts is willing to go, both physically and mentally, to get revenge and kill anyone even remotely related to the God Hand and Eclipse.
