Before I used to trade a variety of coins depending if there was a profitable opportunity, but as time progressed I decided to focus on trading only one coin "Bitcoin", and to be sincere I saw tremendous success with paying all my attention to one coin, and with time I began to understand Bitcoins behavior and they way to react with its price movement.
I also feel what you are saying here. Trading only one pair is really of advantage because it gives you the much time needed to focus on it, you will have time to read your chart and analyse properly to get a maximum benefit of it unlike trading multiple coins just for greed and yet there is possiblity that at the end you lose both multiple trades. To trade a pair and focus on it gives you concentration to monitor it incase the trade is going wrong you can opt out or you may want to adjust your stop loss or take profit parameters. I have witnessed this, to trade few pairs gives better focus than having multiple pairs which effect is greed.