There's a distinction to be made between many different players in 'hft'. You have market makers, latency arb, plain old short-term quant trading which all provide different sorts of 'economic value. Most players are running a mix of some to all of these strategies - for example, in options, fast trading on spot moves is really some weird mix of everything from latency arb to market-making to long-term option pricing.

One could argue that pure latency arb (what flash boys portray as hft) is fairly useless since it's just trying to catch stale quotes that market makers would move otherwise, but that's a very small portion of the industry.