I've been using a 9.7 inch cellular iPad as my primary mobile device for 6 years or so. It works very well for my use case but I do understand why more people don't do it.
In my arrangement, the tablet replaces a phone, ebook, paper notes, and a laptop. I primarily use the machine for messaging/email, maps, reading (books, websites), writing notes, and using terminals. I use desktop computers with many displays (and clouds/racks of servers) for anything "real work" like. A laptop doesn't cut it.
The bigger screen is significantly better than a phone (even a big one) for most of those things. Especially multi-tasking use cases where I want slack+a terminal visible at once. The bigger keyboard is far more comfortable for taking notes and using terminals.
I make/receive very few phone calls, and the ones I do use a headset and a Voip app. If you use the phone a lot for holding up to your head and talking to people a tablet works poorly. Most of my camera use is for things like scanning documents, qr codes, etc where the size/awkwardness is less an issue. I have a single purpose camera that I take on photo heavy trips like vacations. These are both trade offs that make sense to me but I don't think make sense for a lot of people.
I have a case with a hand strap that makes it safe and easy to use the tablet while standing/walking around, something that is hard to do with a laptop. I don't own or use a car so I tend to carry a messenger bag with me everywhere for general storage so adding the tablet to that is straight forward. It is easy to take the tablet in and out of the bag while standing if I need to look something up. Because I primarily use public transit I am free to actually use the screen of my device while in transit. Again, the more common use case where your car stores your stuff and you can't use your screen while in transit likely pushes the common case towards phone sized devices.
I pay $17.50 per month for Verizon cellular data with no contracts and no funny business. LTE data plans for tablets are priced assuming you have a smartphone already. Especially with today's $1000+ phones, a tablet + LTE plan can be drastically cheaper than a smartphone.
Finally, I like the friction of usage balance. It is much easier to grab the tablet (especially while standing/walking) than to deal with a laptop. It is a bit harder than pulling a phone out of your pocket. This leads me to avoid wasting time on my mobile device when with other people, in meetings, standing in line, etc, but makes it much easier to deploy computing power when it is necessary (standing in the data center and need to run one terminal command quickly).
The one thing I would change would be to add a watch that would help with turn by turn walking directions, basic message checking, and NFC payments. I look forward to the day that Apple Watch will pair with a tablet (or work with no tethered device).