After installing the Windows 8.1. Update on my Windows RT device, I was missing the power button on the start page (for new features in Windows 8.1. Update, see windows.microsoft.com/en-us/windows-8/whats-new).
Looking around in the Web, I learned that the appearance of the power button depends on the device type and size, not on the CPU architecture (x86/x64 or ARM) – see Windows Experience Blog.
Also, I found a hint about how to change the registry to enable / disable the power button myself.
But this post states that I should add a new value to the key
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ImmersiveShell\Launcher, which means this change has only effect for the current user. Since I wanted to enable the power button for all users of the device, I added the following value to this key:
Launcher_ShowPowerButtonOnStartScreen, type DWORD, value 1
Now all users of my Windows RT tablet have the power button on the start page.