Every time they use a public wifi hotspot. Any time you use a network you don't control and where you have no reason to trust the admin, you may as well be using a proxy.
The requirement to trust the admin isn't about the admin MITMing you, but rather trusting their competency in preventing other users MITMing you. Of course the admin could be bad as well.
In your local starbucks? Probably very rarely. In the VIP business lounge of an major international airport? I wouldn't be so sure in this day and age...
In the cafe on the corner of your block in syria, iran and similar places though, the odds are a lot better.
I set up a VPN server at home on an old laptop, it works well for me in these scenarios (also for location-blocked services like Netflix while travelling out of country).
