I dont agree with you , if you properly use the multiple exchange for trading or arbitrage trading then it is very profitable, i am using arbitrage trading for some 1 or 2 coins and every week earning nearly 0.02 btc. So it is upto you how smartly you are using it. Even i am trading in multiple exchange with lot of altcoins due to all altcoins are trading according to their listing to their exchanges.
I kind of agree with both of you though. It depends because there are people who will be efficient in trading when focusing on one exchange, there will also be people who are comfortable switching from one exchange to another. I kind of consider myself to be the latter