The collection editor contains features that are dependent on the videos being transcoded. If a video was added that had not finished transcoding, you would be unable to do the following:
scrub the video to specify a start time
grab a poster frame for a thumbnail on the scene menu
see the total duration of videos in the collection
preview the video associated with the button
Also, if there was a problem transcoding the video after it was added to the collection, you would have a video linked from a collection menu that potentially could never playback. By preventing videos from being added to collections until they finish transcoding, we are preventing the client from having a bad experience.