I hate to tell you guys this, but JKR isn't in any way as calculating as you may think... Most likely she doesnt even know the full nature of portkeys. I agree with Bartuc in saying that JKR was just being lazy and looking for a quick fix that didnt leave either Harry or Voldemort dead or badly injured. (It would have been a bad way to end the book) She was most likely looking for convienience and found it in the form of a plothole... I mean portkey...