Most modern PCs have the ability to go into a very low-power sleep mode and can wake easily by input or schedule. I don't know about other companies, but I have our systems set to do maintenance and such after hours and weekends. With that in mind, our PCs pretty much never shut down--no need to have a power button handy. Mobile devices also basically just sleep when not active. And then there's the whole thing of using physical button(s) (combinations) for powering down, as mentioned by someone else. So, other than shear personal preference, I don't see a particular need for a software power button. Personally, I really don't want a way to over-simply swap users and/or restart. That's something I prefer to leave as a task that requires particular intention and specific procedure, not a "click, click... opps!"
Now, I do understand that some PCs will need a simple, fast way to swap users, such as a kiosk or similar massive-multi-user system, but those are special circumstances and will likely their own custom procedures for handling such things.
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