In making music, you might say that there's a slider that you place yourself on, with the two extremes being (attempting) originality and full-blown rip-off. Personally, I get a better feeling when I try to craft a song myself rather than just taking a chord sequence from some other song, but I'm expecting that feeling to eventually fade and possibly be replaced by cynisism as I realise that almost all possible catchy chord progressions have already been used, re-used and recycled.
Of course, going into rip-off land is a bit dangerous, but chords and bass-lines seem to be safe areas.
But for the aspiring composer, it does present somewhat of a dilemma: try to do it from the ground up, or search out previously used catchy chord progressions and work with those? I think I would try to place myself somewhere in the middle, perhaps with some strays towards knowingly ripping something off if it's suitable. (But taking melodies is a no-no. That's lawsuit-land.)
I came across this neat video clip that tells of how a single 6-second drum sample from the 60's was re-used in, well, just about everything.