What advantage do Mt.Gox or Kraken have over Coinbase?
An important advantage they have over Coinbase is that they can offer a service to other people than U.S. inhabitants.

mtgoxUSD may be by far MtGox's largest market, but don't forget that mtgoxEUR and mtgoxJPY are still the 5th and 6th largest markets that exist. More trade volume means more robust and stable prices which is a good advantage as trader.

I'm not familiar with Kraken, so I can't speak for that.

Also, Coinbase does not support limit orders. This is very important. Read the link or search a little about limit order and you will understand.