Your have a point, at least for now. But plans are changing very quickly, mostly in responses to competition from TMobile.
Non-contract plans with large data allowances are now being offered at attractive prices, much better than just a few months ago. My wife and I share a plan on AT&T, recently upgraded (for free) to 20 GB per month for our two phones and two iPads. This is plenty for us to freely use cellular data on the iPads when we travel, knowing that we are unlikely to exceed the allowance, not even close. We can even stream a Netflix movie once and a while; plenty of data allowance for that - plus checking email, reading forums, checking bank account and so on.
Using cellular data can frequently be faster and more reliable than wi-fi. Yes, wi-fi is widely available, but it is frequently slow and unreliable, especially in hotels. I usually use cellular data rather than wi-fi for this reason. More importantly, cellular networks are more secure - I have no qualms about accessing my bank account on a cellular network, but I would never do this on a public wi-fi network.
And things are changing rapidly, changing all the time. I suspect that a year or two from now we will routinely see low-cost cellular plans with 20, 30 or even 50 GB data allowances per month. Plus, Apple is upsetting the cellular carriers' applecart (pun intended) with regard to locking devices to networks. I predict that future iPads will be carrier-independent - you can use them any any network you choose, with no contracts. That means that you can take advantage of promotions and new offers at any time.
I carry (in my iPad's case) an already-activated (and free!) T-Mobile SIM. I activated the SIM so that I can get 250 MB of data per month, for free. No strings attached - no contract. Not a lot of data, but still free. And I can get free data internationally using this SIM when I travel to Europe. What's not to like?
If you want to save a few dollars with a wi-fi only iPad, that's a reasonable choice. But I see great value in cellular-capable iPads and will continue to use them.