HFT increases the liquidity of the stock, and therefore its value (slightly).
The way I understand it, this isn't and can't possibly be true.
HFT "works" by creating both buy and sell orders for stocks and commodities at the same time, and either completing those transactions or canceling the buy/sell orders and posting new ones based on new information, often several times per second.
If the HFT system ever completes a buy/sell order, it only holds the stocks it trades for typically less than a second, meaning there must be a matched bid to buy and a request to sell that stock within that time frame if any transaction is to take place. Otherwise, the orders would be canceled and reposted.
In other words, HFT can only be effective if there are already buyers and sellers in the market at that specific time, and even then only to the volume of stock that is being sold or purchased.
I have a hard time seeing it as anything other than a man-in-the-middle attack looking to shave a few fractions of a cent per transaction. Maybe this made sense before computerized transactions were a common thing and you still had humans negotiating the transactions, but now that *all* transactions are negotiated and matched by computers it seems completely unnecessary.