Well I would consider them a niche market, because not that many people buy them, upon release. Most wait until the price drops, normally after the next niche market 500+ card comes out.
They are for the elite, so to speak that want the latest and greatest, not the normal buyer than wants good performance at a fair price.
The difference is that "few" who buy the cards, when talking about $500 video cards, is several million.
The "few" who buy the TH2Go's number in the thousands to tens of thousands.
But whatever, I'm sure it is coming, someday. I remember people saying 1GB of memory on a video card would never happen.
And I know people who believe the United States never landed on the moon ;). Those who thought 1gb would never appear on a GPU were simply ignoring Moore's law.
The GPU manufacturers have very little to gain by making cards that can output to multiple screens as if they were one (ala TH2Go or "span" mode). As it is current cards with three outputs can't even span three monitors together.
To each his own. Everything's bound to happen someday, I guess I'll concede defeat from my retina-based iEverything (firmware hacked, of course).