Instead of upgrading my phone to 4G, I bought a 4G hotspot, and now everything talking wifi can use it. Count as an advantage that this way no hijackable baseband processor is running inside your tablet, and as a disadvantage that now also entities that only track wifi MAC addresses can track your movements.
(Also an advantage over using the phone for data is that by using a data-only SIM, your ISP won't do some nasty stuff like blocking VoIP. In the end, I may go back to a dumbphone, too.)
I used navigation as an example. In general though I really don't get the idea of wifi-only table. I commute regularly and get most of my non-coding work done during that time; e-mail, responses, catching up on news and a whole bunch of things. Also when I'm out I carry my tablet around, just in case.
And I have a dumbish phone which I use for making/receiving calls :-)