The problem with SMF is that if 2 people were to sing over the top of them (on the same keyboard, just the SMF with no parts added) you would get 2 different vocal performances with 2 identical backing tracks. On the other hand if 2 people were to play the song live on the keyboard, you would still get 2 different vocal performances but obviously you would now get 2 different interpretations of the song. That's why I don't like SMF's myself. There is no room for expressing yourself live when the song has already been pre-recorded.
It's each to their own, there is a market for the 100% live entertainer and the musicians that still have to rely on backing tracks/SMF/MP3 etc.