Nick Jonas composed the music for Wintertusk back when it was released (in 2011, I think). His NPC is still in the Grizzleheim area conducting an "orchestra" of the music he wrote, with samples of the soundtrack from each area of Wintertusk.
It's just a leftover promotional stunt, mostly. The licensing agreement must have allowed KI to keep his NPC there permanently.
In contrast, Selena Gomez guested as an NPC during the summer of 2010 to promote her album: A Year Without Rain. We rescued her from King Thermadore, who had locked her up in Crab Alley so he could recite poorly written love poetry at her all day long. At the end of that summer, those quests were removed and the only trace of her adventure in the Spiral is, some old-time wizards may have her portrait or a statue of her in their house.
He is there because he helped with the music for Wintertusk. It let people know about the launch of that area, and gave us a listen to the music of that area when it was new. Why is he still there? He's not doing any harm, and new pirates that get there still hear about Wintertusk from him. Will he ever have anything new to say? Who knows? I guess you can consider him a musician stranded in Grizzleheim, waiting for his ship.
He created the music for Wintertusk. So even though he doesn't have any quests for you and there's not a storyline about him like Selena Gomez, he's still there "conducting" the music that you can hear by pressing "X". He's basically just there as a reminder, I think.