I've read on a kindle very close to every single day for at least 10 years (Kindle Touch first, now Paperwhite), and I keep it in airplane mode 99.999% of the time or more. If I buy a book from Amazon, I'll put it on Wifi until it's downloaded, then back to Airplane Mode.
I know that I can look something up on Wikipedia were I online, and this is the only feature I know of that I might sometimes be tempted to use. It's difficult for me to imagine a scenario that I would want to buy a reading device that could connect to the internet on cellular data.