A Valve engineer has revealed on Github that the suspend/resume function planned for Steam Machines has been cut. The problem, he explained, is one of reliability: "Given the state of hardware and software support throughout the graphics stack on Linux we didn't think we could make this reliable."
The suspend function puts hardware in a state of not-quite-off, which saves power but allows machines to carry on with essential, under-the-hood tasks, and also makes firing them up a much faster process. That second part, the "resume," is where the trouble lies, according to this Slashdot comment, which digs into the issue in considerably greater detail.
"It doesn't probe and reattach the controllers to the same point in the device tree that they were in when the system was suspended. Since those are the device nodes that SteamOS has open at the time of the suspend, and they route to The Noplace(tm), the controllers become unresponsive," user tlambert wrote. "This is a general problem in the Linux device model, and you can see problem in the device model poke their heads up in various places."
This probably won't be a big deal for most PC gamers. My PC has a sleep feature, but it only ever gets used when one of my cats steps on the button. But as Ars Technica says, its absence may be more pronounced on hardware in the living room, where Steam Machines are meant to be, because they'll either have to be left on all the time, or require a comparatively lengthy boot-up process whenever they're turned on. In some cases, noise may also be an issue.
As for why Valve doesn't just fix it, tlambert covers that base too. After a lengthy and technical explanation laying out the depth of the problem, he wrote, "Really, the Linux device management system for input devices needs an overhaul; unfortunately, doing this crosses too many kingdom boundaries, and there are too many stakeholders involved to really get traction, unless you happen to be one of the area 'owners' for one of the involved subsystems."
The initial Steam Machine lineup appeared on Steam in March, but won't actually go on sale until November.