Well, why not add it? :p
The list of changable overlays is really long. I was surprised to find that Dew Point wasn't included. Dew Point is, after all, one of the most essential markers for the condition of the air around you.
This might be very subjective, but the window for what people find comfortable and uncomfortable is a relativly small range, usually 10c to 20c. Temperature, on the other hand, has a much wider range.
Plus, Wetbulbtemperature has it's own listing, why shouldn't Dew Point have it then, it's used much more (I would guess) :p
Best regards :)