support $elemMatch in aggregation $project operator

Why can't $elemMatch be used in the $project phase? It would allow me to easily select an element out of an array.

Example:

db.person.aggregate([

  { "$project": {'main_event' :

    {'$events_array': {'$elemMatch' : {

      'event_status' : 'main'

    }}}

  }}

]);

Basically what I'm looking for is an easier way to convert an array into a scalar without having to $unwind and then re-$group, which is taxing for proc time.