"Missing something?" The other benefits that come with the cellular version. The tangible difference between the wifi and cellular version is not limited solely to having a built-in cellular data connection.
And guys with iPhones are biasing the answers. Does OP have an iPhone to do what those guys are doing? If not, his benefits of a cellular model could expand well beyond the tethering iPhone data scenarios. For example, I bought an iPad Mini with cellular INSTEAD OF an iPhone. My needs were data, text and only a little voice, and cellular data only when traveling. I covered the voice to a "good-enough" level with VOIP (mostly talkatone & Skype, plus earbuds with microphone). As a result, instead of paying $1000/yr for phone service, I've paid about $60 so far this entire year for just a couple of rounds of cellular data (all the rest of the time leveraging free wifi).
If OP has an iPhone and can tether, his "best" (defined by him as "cheapest") may be to do exactly that. However if he doesn't have an iPhone, an iPhone-oriented solution for him is definitely NOT going to meet his criteria for "best." In fact, that's probably the worst (most expensive) way to go (per his definition of "best").
OP, if "best" = "cheapest" it's probably a used iPad (or just use the one you have now) that is good enough for your needs (which could be up to several generations old to help minimize price) and a used mifi device that runs on the cheapest service in your area. You'll probably want to heed the advice of service reliability in choosing that service, including potentially NOT choosing the cheapest if it has bad service in the places you will go. There is some convenience cost in de-coupled cellular (juggling 2 devices vs. 1) but that is probably THE way to cheapest.
And consider downloading an app like the one from jiwire and simply leaning on using it to locate the nearest "free" wifi if you don't absolutely need the extra convenience of a cellular connection. There are lots of places that offer wifi and apps like that one will identify those near where you are or where you are going to be. Maybe you'll find that nothing is so urgent that you actually need a cellular connection (and the cost of it).
I don't buy into "best" = "cheapest" myself but I am a guy spending about $60/yr in total on his cellular plan for data, text & voice. Consider all of your own uses- and the details of quality of service in the places you go- and choose wisely.