I can certainly see the appeal of that 'always-on' connectivity, getting notifications when you're out and about, and have considered it in the past.
Unfortunately I think the price for a cellular iPad, plus the cost of a SIM plan mean you actually need to 'need' that functionality, it's too much to pay for a 'nice to have' feature imo.
If cellular was £100 more and there were more decent data only sims (tbf I think you can Get a few gigs for a fiver a month) then perhaps. But Apple make tethering so easy you don't even have to get your phone out, maybe in the future.