Adding Sebastian to the game was really a narrative necessity, given that the base game was noticeably lacking a character to lend some stakes to the whole subplot in which the Divine tries forge a middle ground between the Templars and mages. But the character himself is such a bore that the narrative dividends never really materialize (this point is perfectly crystallized in the goofy scream Sebastian gives after the Chantry blows up - it's a genuinely funny moment of unintentional hilarity, when they were clearly going for pathos).

But genuinely, there's really nothing to recommend about interacting with this added character. His main concept is that he's pious, which in this game means he mopes a lot and petulantly complains about the Maker's failure to strike down his enemies. Then, when you finally do that for him, he starts thanking the Maker. What an asshole. There's also a chaste romance that female Hawke can pursue with him, which might have been interesting in a game with better writing than Dragon Age II. Add in the fact that the quests added here are basic dungeon crawls with no significant gameplay innovation to make them stand out, and the idea that they charged money for this becomes more than a little vexing.