Many stocks are completely uncorrelated or even negatively correlated providing huge advantages in diversification. It's amazing how under-appreciated proper diversification is. To my knowledge there are no futures or FX with auction orders, which means you are forced to have slippage and face relatively high minimum ticks.
For most traders who get into FX and futures, the attraction is purely about leverage.
Is this what you are referring to?
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Auction Orders
An auction order is entered into the electronic trading system during the pre-market opening period for execution at the Calculated Opening Price (COP). If your order is not filled on the open, the order is re-submitted as a limit order with the limit price set to the COP or the best bid/ask after the market opens.
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I see no advantage to use auction orders if you are trading futures intraday. If I want to avoid slip I just place my limit order in the que.
Yes, investors should seek diversification. The question was about short term trading. I applaud your ability to concentrate on multiple markets/products and trade them successfully. Me? I have the attention span of a Hamster so in my case it pays to focus on one product.

(on edit- it depends on what your definition of "short term" trading is. My definition is 5 to 20 minutes.