While usually the specials is pretty much irrelevant to the main story, there are cases where special episodes are a significant part of the series (e.g. Doctor Who or SG-1 / SGA). So it would be cool to have them in the watchlist, arranged by airdate with the regular episodes. So you not only could mark them watched, but also could see in which order they should be watched in binge.
Or this feature could be implemented as part of Alternate episode order feature.
