if my phone is stolen, what stops someone running up a huge bill that I'm liable for?
If you run out of data, you have to phone up and add more on. you can't go over without knowing about it.
They could phone overseas I guess, but there's a cap on how far over contract you can go, presumably you'd phone up and report your phone stolen as soon as you noticed anyway, and frankly most phone thieves take the sim out straight away because they are after the handset, not your account.
We live overseas and WhatsApp is great for calling family for free, sending them texts and photos (DS is the first grandchild in the family), and for work group chats.
I think of WhatsApp as a cross between texting and twitter. DH is in front of a computer all day so he uses Slack for work chats, but as a doctor I am on my feet most of the day, so my phone is much more convenient. And yes there is some trivia, but messages like "xray meeting cancelled", "has anyone seen the ultrasound machine?" and "who's teaching the SHOs today? Nobody's turned up" are actually pretty useful to receive straight away. Not much use getting them six hours later when I get back to my desk, and texting fifteen consultants individually would be bloody expensive, and take forever.
I can also send status updates that don't need a response ("Dr X is absent today, I'll be covering clinic") and know that everyone's seen it - our work email inboxes get completely clogged up with rubbish (hospital on black alert, IT downtime, pathology mailshots, various all-staff spam from HR/PR depts etc.) that we often miss important things.