Artists seem to have always used the possibilities of the actual support the music is delivered on. In the vinyl era many recordings made use of the fact that there were two sides to a record and obviously the B-side of a single belongs to a whole music genre on its own.
The arrival of the CD brought along the hidden track. The first time I really became aware of this was with Nevermind from Nirvana, which I had bought in vinyl only to find out later on that the CD version actually had a bonus track (Endless, Nameless).
On the recent Jarvis Cocker solo album the song Cunts are still running the world is hidden on the very end of the CD, at approximately 30 minutes of the the last song (Quantum Theory).
I actually quite like this song, but of course it’s a pain in the neck to fast-forward the CD player for quite some time in order to hear it. Also the long gap means I have to pull the ipod out of my pocket to fast forward, unless I want to enjoy 25 minutes of silence.
There are probably all kinds of good artistic reasons to “hide” tracks, but I won’t miss them as the disappear in the download era. Unless people are happy to download 20MB to have two songs and a long silence in between.