As far as data use is concerned, I almost never use anything close to the 5 gig ceiling I get with the hotspot. But last week I was on a business trip where the "free wifi" in the hotel was virtually useless. My mobile hotspot gave me 4G/LTE speeds (5-9 mg/p/s) for my laptop and my iPad. Streaming video and audio were no problem at all.) And over the course of the seven days I used about 2.9 Gig of my monthly allotment. In short, it was a lifesaver.
Your situation is obviously different. But frankly, you're exactly the sort of customer that Verizon (and other carriers) are committed to eliminating from unlimited data plans when you try to add new and more data hungry devices like the iPad. As you note, perhaps you're unlikely to increase your data use when you tether an iPad to your phone. But Verizon knows many users will begin to stream video to their iPads when they didn't even consider it on a phone with a tiny screen. To limit the demand on their network (and obviously to increase their profits), Verizon and their brethren want you to purchase a 3G iPad with a separate (limited) data plan or pay an outrageous premium to tether your phone.