Joysticks can do this, too, but their DPI is frequently nowhere even close to acceptable for this kind of work - they tend to be somewhere in the 200-400 DPI range, when your average mouse tends to be capable of input at several thousand DPI; if the touchpad resolution is good enough, then I think they're going to have a winner on their hands.
The problem is that you can't.
There are just too many different type of gamers having different requirements. Given Valve's lineage, their core audience are the FPS/PC type gaming crowd who are used to keyboards and mice. And this controller makes sense for that audience.
While I'm interested in the Steam Machines, I would never buy this controller for myself, because I'm into fighting games and old-school console type games. And even though the Steam Controller isn't for me, the great thing about Steam Machines is that they're still computers at heart, which means that I can probably use just about any other controller out there if it has USB connectivity and identifies it as a standard HID.
As a PC gamer, I'm super-excited for this gamepad because it will beat the ever-loving-crap out of a joystick for games that depend on a pointing-device (strategy, RTS, no-aim-assist-FPS games). It might not measure up to a mouse, but it could still provide a "good-enough" experience unlike a thumbstick.