Did that for a while with my iPad 2. Hated it. One of the pleasures of taking your iPad mobile is how quickly you can start using it. Reading the forum on the bus, checking your social networks for a few minutes at the park (in the middle of a walk), or even browsing while you catch a meal at Subways.
All of these things become a hassle if you have to take out your iPhone, turn on the hotspot, then wait for the iPad to connect. You might just as well settle for the smaller screen on the iPhone.
The hotspot experience with an iPad is a lot like the old laptop and hotspot combo. Fine, if you are going somewhere without wi-fi to sit and work for several minutes to an hour; but too much trouble for quick, casual usage.
That would go double for an iPad Mini. What's the point in having an extra-portable device that can't be used anywhere you go, instantly?
To me the extra $130 device cost and extra $10 per month carrier fee (to add it to my existing data plan) is well worth the savings in time and convenience.
Not that I expect every one to agree. Different folks, different. . .