Many people have videos presented in 4:3 aspect ratio (such as old recorded tv programs). The ability to select a border (or image fuzz duplication, or poster, or stretch image) in place of black bars would bring Plex ahead of how tv stations handle 4:3 content on a 16:9 display.
Edit: Examples:
current view (default view/fit)
Fuzz duplication example (common for TV broadcast)
Stretched image, same as how DVDs can display anamorphic video
Poster display, because we have posters from IMDB/TheTVDB
Edit 2: I have been made aware that the web browser can fill - would be nice for couch viewers using Roku/Xbox/Playstation though.
