Currently Mongoid's map also can perform the pluck operation, by being called with a field name. This behavior was added in a9cff701b2 and it looks like the author was unaware of pluck because pluck was actually added about 5 months earlier, in 441abe20130639e97b38f9ac216af455a24dbba8.

Since there is a dedicated method for plucking, the plucking behavior should be removed from map. Doing so will make the signature of map identical to both Ruby core and AR (neither of which have plucking behavior in their maps).