Before I got a keyboard case, my iPad was a "toy".
After getting a keyboard case, it replaced my notebook computer.
An iPad plus a keyboard case may be bigger than an iPad alone, but it's still *WAY* smaller than any laptop, even the sveltest netbook. (And generally much more 'usable' than a netbook.)
Yes, I can type on the on-screen keyboard. And better than many can. But not for long periods, and certainly not "eyes-up" touch-typing. You try typing notes on the on-screen keyboard while looking up at a speaker/screen... I can actually stay fairly on-target for a sentence or two, but past that, my fingers have drifted far enough to be unintelligible.
Plus, many/most keyboard cases aren't significantly bulkier than a just plain case. I use the Zagg aluminum-case keyboard case for the original iPad, which is the same physical size as their non-keyboard version. (And only cost $15 more than the non-keyboard version at the time.)
That said, the iPad mini is small enough that I don't think I could touch-type on a keyboard that small. I have reasonably large hands, yet I can type just fine on a full-size-iPad-sized keyboard (as well as on a 9.8"-screen netbook's keyboard.) But anything smaller than that and I know I'd fat-finger often. Just looking at the video of the iPad Mini Mod keyboard, that person looks to have fairly small hands, yet you can see that they 'double-press' a couple times.