Actually, I think the argument is to address the major short-coming of the iMac.
That is, that if you need a larger screen, that you must buy another computer.
Say I desperately need a computer for school or work. And, the 17-inch iMac is absolutely all I can afford. And, to buy the 17-inch takes every bit of money I can come up with.
Then, a little while down the road, I need a bigger screen. Well, now I must either put a second monitor next to the computer (which may not look nice at all), or get rid of a perfectly fine machine to buy another one just like it with a bigger screen.
If all I need is a larger screen, throwing-out or getting rid of the computer just to buy an identical computer with a larger screen makes absolutely no sense.
Putting another monitor next to it, usually results in an ugly combination.
So, you are typically faced with the choice of something ugly, or wasting a good machine to buy another one with a larger monitor.
Most would probably opt to just go with the ugly option since buying an identical computer and losing some money in the sale of your current one just doesn't make sense.
But, that defeats the idea that is promoted with the iMac of simplicity and beauty.
So, with that in-mind, an add-on screen that would compliment the iMac's appearance would have a likely market.
I know I once had an iMac G5 17-inch. But, later wished I had a 20-inch since I couldn't fit all my stuff on the screen. I set a PC monitor I had (an LCD which was very attractive by itself) next to the iMac, and the clash was just ugly.
So, I lived with the lesser screen space because my budget did not permit selling the iMac at a loss just to buy the same machine again with a larger screen. And, the clash of the two monitors was just plain ugly.
So, while some may not think that there is a viable market, and that everyone should just buy another computer when they need more screen-space, it should be understood that not everyone sees that as a logical move.
