For most players, Marlin was one of the most annoying bosses because, he punished you for being good at the game. But in the exception of Maru who had the ability to decide which ability he wanted to be shot with, he still decided to choose the arc because, if you kill Marlin you get whatever item he shot out.
The arc was the most valuable because, it had the ability to erase anything or anyone and it has 3 uses.