I have a theory about this auto-exchange and I would like to see your opinion about this. Let's take for example that we have all coins to auto-exchange
So the best would be BTC or ETH to be set as auto-exchange coin (I would choose ETH because have lower withdraw fee from MPH)
In your example when BTC goes up and ZEC doesn't change price you are not losing anything as you will buy more LTC for your BTC. You have only considered one case, but there are many more options how the price of one or the other coin will change. In some cases you lose, in others you win.