I am in south central Oklahoma. I find my cab tractor (39.5 PTO HP) quite useful.
In summer: When it gets hot and humid I look for things to do with the tractor instead of being out in the heat.
In winter: My little 4x4 Kubota will get me there irrespective of the conditions to feed the herd and break ice on the ponds so they can drink without my having to bundle up to the point I can hardly wiggle. No more getting rained on and having to change clothes 2-3 times a day.
Mowing etc without getting dust covered. Spraying without getting sprayed. Less insect problems (mosquitos, hornets, bees, wasps, chiggers, ticks)
What has size and HP got to do with it except you need enough HP to run the A/C. I see no difference in motivation for a cab between large and small tractors. It isn't a size thing, it is a use thing. If you only use your tractor for 30 min to mow the lawn or have the luxurious option to defer any given tractor task till inclement weather has passed or you don't mind getting baked in the sun, frozen in winter, rained on and so forth then a cab is no big deal. When I operate someone else's tractor, sans cab, I have to try to remember the limbs and such can swat you.
I have obligations to my animals and they have to be taken care of no matter what the weather. A cab tractor makes that much easier on me. I don't like to let a project hang too long because of weather if I can avoid it. A cab helps, quite a lot.
If tractoring is something you do only when it is fun or you are in the mood and it is convenient then a cab is not such a big deal.
As far as the manufacturer amortizing his cab design and testing over a smaller number of units and such... I don't worry about it since I can't effect it.
If I had to pick between a cab and a few HP (due to $), I'd go for the cab. If that meant I had to operate the tractor a little longer due to less HP, at least I'd be more comfortable and safe. Long hours of tractoring will take the starch out of your sails, with or without a cab but with a cab is way nicer.
Pat