I think where this gets messy is whether the status of a video game or anything else as "art" (in this case a game designer, or group of game designers) means that the artist or the audience should have the final say on how the narrative takes shape. Ray Muzyka's statement about ME3 came down on the side of the artist, arguing that the game ended the way its creators wanted it to end and that therefore the ending should not just be discarded and rewritten altogether because fans didn't like it.
In most examples of what we *would* all agree to be art, I think that's exactly correct. Da Vinci didn't convene a focus group or a test audience to decide whether the Mona Lisa should be a blonde or a brunette. On the other hand, if something is clearly a product - say, a bottled beverage - and 99% of taste-testers say it makes them want to vomit, it would be crazy to ship it anyway on the grounds of the beverage-maker's "artistic integrity."
When push comes to shove, I'd have to say I would side with Muzyka in the case of ME3 and video games in general (which is not to say that game designers shouldn't listen to constructive criticism). Yes, games are products that we pay for and use for our own purposes, but so are movies, music, TV, books, etc., and games are closer to that end of the spectrum than they are to drinks, pieces of clothing, furniture, and the like. That's not to say that we have to *like* all "works of art" - I can sit down to read a novel and decide it's worthless crap, but I still wouldn't say that my opinion, even if widely shared, means that the novelist should be required to rewrite it if (s)he honestly believes it should remain unchanged.