Yep, if you're tethering your iPad and using it enough to cause iPhone battery problems or hit a data cap, then a cellular iPad might be for you. Otherwise, I feel it's a waste of money. I've supported a LOT of people who insisted that they needed it, and ended up never using it more than once...
Academia frequently uses cellular iPads as well, for collecting survey data in field studies. For personal use, as Ron points out, just tether to your iPhone. Works great.