If you use Windows 8 on a touchscreen device, you'll notice that the keyboard is somewhat unusual. Unlike most conventional keyboards, it doesn't have a row of numbers, and it doesn't have a Tab key either. What gives?
Windows 8 isn't the first touch keyboard to put numbers on a secondary keyboard (it often happens with mobile designs), but the explanation on the Building Windows 8 blog goes into more detail about that absence:
Some of our early designs and prototypes had a number row too. But when we brought these designs in front of people, the feedback was strong that the keyboard felt :cramped" compared to what they were used to. We observed frequent errors and accidental invocation of keys, especially around the perimeter of the layout. This resulted in a number of changes, and it confirmed the decision to not include a number row. Here’s why: Including a number row meant adding a fourth row of character keys. When we optimize for keys with a targetable size, that means the keyboard must be that much higher. On a typical tablet device (say with a screen size of 10.6 inches) adding a number row would mean that more than half of the display would be covered by the keyboard. When we combined this with the observation that numbers are typed less frequently than most letters and common symbols, and you recognize that the extra keys are causing accidental key presses, we settled on including numbers on the separate number and symbol view.
A similar logic applied to the Tab key, which was on some of the early prototypes for Windows 8 but was also eliminated to reduce errors.
Both options remain in place on physical keyboards of course, and for dedicated keyboard shortcut junkies like me the touchscreen keyboard is not going to be a place to spend much time anyway. But if you're interested in how interfaces evolve, the whole post is well worth a read.
Designing the Windows 8 touch keyboard [Building Windows 8]