I feel like it's really important to include Spearmaster's scar because it's a consequence of the tragedy of the local group inflicted on an innocent messenger, but it also marks a change in the relationship between Spearmaster and Seven Red Suns. NSH points out that Suns expressed nothing but disgust while developing Spearmaster, but it's after Pebbles tears the pearl out of Spearmaster's chest and leaves them scarred that Suns realizes how much they've come to care about Spearmaster.
It's just one of those things; the scar is a sign that time has passed. The scar is a result of a grievous wound inflicted by Five Pebbles' in his rage, but it also lets us know that the journey is over. Spearmaster and Suns have found peace together despite the circumstances of the local group.