I'd have thought a large majority of what's app users use it for chatting. I can't imagine they're particularly fussed about people sniffing their plans for meeting up that night. There are varying requirements for security...
It isn't the information they can view, it is the things they can do impersonating you. Any application installed on your phone can probably access the two authenticating pieces of information. Then they can impersonate you in messages to, say, your parents and say something like "Hey mom, I need to order something, can you send me your credit card?" and then your mom, under the illusion that WhatsApp is secure, will send it right over.
A large majority of whatsapp users use it as a straight-up replacement for normal SMS. It's easy to impersonate someone's account and pull off the whole "I am stuck please wire me money" scam.
I suggest you go find some nearby open wifi in use, spy on some people, then tell them what you've found and see how they react to it. Report back when you're done... If you can....
If the table next door was having an obviously private discussion they'd probably be a bit put out if you started offering your opinion. People know their conversations often aren't secure, but then there are certain social expectations. Sometimes it's just expected that other people will politely ignore their conversation. If you told people that you had been spying on them I guarantee you that they would not blame their tech. They would place the blame squarely at your door for having listened in on something you shouldn't have.
