As far as i have experienced they make profit in relatively slow or ranging market only during high volatility they make loss trades and in the end investor has to either switch it off or loose all his money, so personally im not a fan of bots.
I have used bot to trade once and I couldn't tell the story. Bot can only generate profit for one who kows how to use the bot well. For me it's a no no. What I don't understand is how the bots are used in some exchanges. It still baffles me.