Rather it be optional considering a lot of games have had crummy DLC "expansion"-like packs for story content which ended up being a waste of money or ignored once the new game came out.
It also depends on what genre the game is in because it works a lot better with RPGs and MMOs than something like a fighter or shooter (not counting the Half Life series).
I did enjoy the DLC episodes of GTA IV a lot more than GTA IV and while it wasn't a bridge for a new GTA game, it did well bridging the connections to the main story from different perspectives.
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