You don't have to pay $120/yr for an extra line. The cellular feature does NOT require a contract or activation. If you don't want to use cellular data, you just don't sign up for it.
The nice thing about having the cellular modem is that you can choose to turn it on in a situation where you find you need it. AT&T, for instance, offers one day of access for $5.00, or 1GB you can use over 3 months for $25.00.
I travel a lot, and many nicer hotels still charge for WiFi access, on the order of $10-15/day. These plans that you can turn on and off at will are often cheaper than paying hotel rates. I also use my iPad internationally, and it is nice to just pop in a local SIM and be connected as you move around your destination, without having to hunt for WiFi hotspots. iPads are not locked to a carrier, so no need to jailbreak or any other tricks. In Germany, for example, I bought a Vodaphone SIM for 4.95 Euro, that includes a couple hundred MB of data good for a month (worth 4.95 Euro, so the SIM card was essentially free). An additional 1 GB was 14.95 Euro, which was plenty for the week I was there, and less expensive than paying for my hotel's WiFi access. Calling home over FaceTime was also essentially free at that point.
I use my iPhone 6 a lot, but my iPad Air 2 is also usually nearby, and I would rather use the bigger screen for surfing, e-mails, and anything that involves staring at the screen for prolonged periods.
The moving maps enabled by the GPS are also very useful in unfamiliar locations, whether you use the built-in Maps app, Google Maps, or other apps. I have car navigation apps that download all the maps (Navigon) to the phone. No data connection needed after the download. This saves me from needing a separate GPS unit when I rent a car. I don't want to carry an extra device and charging cables, and I don't want to pay extra for a navigator from the car rental company.
