I think MMOs can get away, by and large, by not having much of a plot. The buttershovels example is good, and something that would be nice to see, but "two armies fighting" is classic for a reason; the game is focused on giving engaging gameplay and views plot as an additional element. If it's thousands of times more fun than adding numbers in Excel -if you are a distinct character in a beautiful realised world interacting with other characters, making decisions and having enjoyment on a personal level- you can leave aside some additional plot.
Funnily enough, it's like connection issues: So long as you can do big, fun things there's not a lot of value in worrying about the small, fiddly bits. It's only when people fail to find enjoyment in the grand schema that they complain about the generic nature of the plot.