One Song Mastering
One song mastering is really nothing too special other than the engineer has the benefit of only having to worry about that one song that will stand on its own. Without the need to balance one song with a group of songs in an album. That gives freedom to give that one song all the glory without worry about out shinning other songs.
In experience I have noticed the difference of a song as a single an the same song on an album. The song as a stand alone single was dramatically more intense than the cut on the album. This is due to the fact that one song shouldn't out shine the others when in play on an album. It would be like those TV commercials that come on that are so louder than the program you were watching that you have to turn the volume down then back up.
Another aspect of one song mastering is that this may be the song the artist wants to release first before an album, therefore the engineer has the flexibility to help the artist make a great first impression with one song without having to humble the song to have unity with any other songs.
Of course now a days this may not always be the case with all the improved technology for mastering out there. In a closing opinion why not just stick to making each an every song the best they can be no matter if it is a single or an album.