They’ve done limited time or limited area gimmicks before. McRib comes to mind.
Maybe they're used as an occasional draw now, to remind people that generally overlook McD's to bring them back in, but I'd bet McD's got a great deal on pork one year to make it worthwhile to produce.
The real gimmick this year was Burger King's limited Valentine's Day "adult" meal, IMHO.
That would be a great marketing case for business schools - to what demo are they appealing???