They did in the past, but at least for Part&Parcel, they felt obligated to modify the map before adding to official coop pool AND there were (and still are?) issues for some time with many maps (regardless of them coming from Blizzard from the start or no) like timing AND must take in account mutations, and nowadays even prestiges.
So unless when Microsoft take control, they give hints to be ready to invest significant time and effort in SC2 and/or co-op, it will probably be unrealistic to count on it.
Sad but true.