If someone copied my sites' designs, I don't think I would be too bothered as long as I wasn't materially harmed. I can think of two possible ways I might be, though, that don't really apply to, say, torrenting movies:

1) Since the two sites would look the same, I could be mistaken for the copycat.

2) My users who go to both sites could be confused, possibly thinking they're related.

Those are two differences I can think of, though I don't particularly support torrenting, either.