The "outro" isn't only designed for the purposes of radio play. Often times it's the culmination of the song's buildup and is meant to have the most impact. Think of U2's "With or without you". The outro on that song is by far the best and most emotional part.
Other times the outro is the chorus (with all the bells and whistles thrown in) repeating over and over until it fades out - giving the sense that the song never really ends. This approach is much more pleasant, in most cases, than a hard, abrupt ending. But of course it depends on the song. Note: DJs talking over a song's outro is one of the most irritating things about pop radio. While it's understood why they do it - to keep things moving - it's frustrating when they're yapping over what in most cases is the best part of a song. Even worse - often times the outro or "vamp" is cut off all together by station programmers so that they can squeeze more songs into a playlist. Not a big fan of that practice either.