As has been stated in other threads, Garageband (GB) does not give you the ability to pull in a Final Cut video file, along with it's markers like SoundTrack (ST) does.
ST is made to make it easy for video editors to make quasi-original, royalty-free background music.
GB is made to make it easy to write songs.
That said, I hope that they add the software instrument part of GB to ST in the next update - what's anoying to me (a new ST user as of 2 weeks ago) is the inability to create drum fills and the like.