The conventional wisdom seems to be that McRib doesn’t sell well on a long-term basis (compared to McDonalds’ permanent menu items), but people who love it really love it, and it works extremely well for them to use it as a limited-time item, which gets them a lot of PR and buzz when it comes back.
And, they usually run it during the 4th quarter, because McDonalds’ sales are typically a little soft at this time of year.