Worthless if you are trying to build your own HFT system perhaps. But not so worthless if you can reverse engineer critical parts of the code and demonstrate that its purpose is to front-run other people's trades rather than just being really fast. If you can show this, you can make a very good living testfying in civil court cases on behalf of clients that got screwed by Goldman Sachs.
That only works for clients of Goldman Sachs. It doesn't apply to the stock market in general. Because HFT is regular trading - by the time you are notified of the trade, it's already happened.
You don't need HFT to front-run a trade - if a client says they want to buy one hundred shares of XYZ Inc., you as the brokerage could front-run that yourself. You always could, and it doesn't take a computer to do that.
HFT just trades really fast. Once a trade takes place, it's broadcast to everyone who adds that trade to the algorithm. But once you hear about a trade, it's happened. The only way to "see into the future" is inquire into the bids and asks queue which will show you the most a buyer is willing to pay (and the amount they want to buy), and the lowest a seller will sell for (and the amount they want to sell). This spread is where everything happens. In an ideal world, if you want to sell stock, there will be a willing buyer at the price you want, and vice-versa, but if the bid-ask spread is high, then your stock is a lot less liquid - either you have to dump it because the bids are low, or you have to overpay because the asks are high. (Remember, you can't just sell stock - you only put it up for sale. The trade happens when buyers and sellers come together and agree - i.e., the buyer is wiling to pay the seller's price, and the seller is willing to accept the buyer's price).
Now there are isolated incidents where trading centers get confused and you get arbitrage happening, but that's a normal behavior as well - surely you must've thought about buying up a bunch of product that doesn't sell in your area, then reselling it where it's constantly sold out.
So many people don't realize how the stock market works, which is a shame, because the stock market is just like any other market or store. Just because you "sell" something doesn't mean it'll sell - all you did was put it up for sale. You can ask anything, but it's up to the buyer or seller to accept.
All markets work the same way - even eBay. Putting something up there doesn't guarantee a sale if the buyers feel the price is too high. Even "sniping" isn't a bad action - it's just putting a bid close to the end time of an auction to try to get the item at a price close to its current bid. But if someone put in a bigger bid earlier, that prevails.
In fact, there's a very accessible "stock market" that with some patience, you can earn a few bucks without any investment. Go get a Steam account, and wait for a sale, and do whatever you can to get cards. Then sell those cards In the marketplace. The marketplace works just like a stock market complete with bids and asks, and the Valve trading server will perform the sales as buyers and sellers agree on a price. You can see trendlines, volume, etc., and learn a lot. And it won't cost you anything - you can easily make $2-5 this way, which isn't a bad way to go for an education in how markets work.
You can experiment as well - sell too low and the trade happens immediately because you'll have buyers. You can be a buyer and put in a bid and see how long the bid takes to be fulfilled.