It's not irrelevant because the parasitic HFT siphoning trickles down thru all markets... stocks and futures alike.
Order-flooding and bid-lifting prevents guys from filling 100 shares, 1000 shares, 5000 shares at their targeted prices. Some of those orders never fill because price moves away & outside the targeted fill zone. What would have been true liquidity in the market if those stock orders were all filled, is instead a liquidity net-drain due to HFTs spoofing and moving price artificially from the true bid/ask market participants while working to front-run order flow.
Day after week after month after year of that shit, guys either toss the towel or scale down size by force. Today's news talked about ICE and NYSE downsizing due to dwindled trading volumes. Oh really? And why is volume down at all? See the HFT chop-churn examples above.
Now over in the futures markets, it's a trickle-down mess. Russell 2000 emini futures are flying all over the charts on 1, 2, 4 lots trades. Try filling a measly 10-lot and see if you don't have your ass handed to you with only 2-3 contracts filled on the runners, all 10 filled in the chop and the stops all slip to boot.
Why? Because liquidity is nil with algos hammering the tape. Used to be nothing to click in 10 - 20 Russell contracts at a clip and not slip at all and fill them all. Now if you click in a 20-lot, you are the Russell market entirely for several strikes.
Any individual trader who is not defending HFTs because they are part of the front-run gravy train simply don't understand what the parasitic effect is on overall markets. It's to the point where you can't get filled on size unless you accept brutal slippage in, out or both in any but the most liquid markets (i.e ES). Guess who is siphoning that slippage while dictating whether you fill size or not?
If all HFT trading were shut down tomorrow, all markets would return to much more normal and liquid behavior in one helluva hurry.