I honestly think if Thornton wasn't a fighter, most fans would judge his playing ability to be slightly above average for a 4th line LW. He skates hard, does a good job of hemming in the other team deep in their zone, gets back defensively, rarely turns the puck over, etc. I never really get nervous when the 4th is out there - they don't give up any more goals than the other lines.
I do have issues when they are out there along with a Ference-Johnson pairing, but that's another argument.
You can't just take "next best talent" and throw them out there in order of pure ability. Lines need balance and chemistry. He's just fine where he is, and putting Caron or Daug on that line, for example, would lessen the overall effectiveness of the line even if it puts in 4-5 more goals a season.
I'm not a huge fan of Thornton, but do think he gets a bad wrap because of the fighting.