Memory is a commodity part. If all vendors follow the logic that they should just upgrade to unnecessary quantities of memory just because "it isn't that much more", then the price difference will no longer be minimal.
And that's if it's even minimal to begin with. We have no idea how much Apple pays for RAM.
I am only talking about this particular model. It is clear that more memory will be needed as the current apps and new apps appear and technology enables us to do more and more.
As we now know the memory is soldered on, with no way to upgrade (But, I am sure some 3rd party will figure that out), which makes an 8 GB iMac a DOA in my book.
However, this model may just be targeted at an entry level user (For that it should be $ 999) with very basic needs.
Over the years I have changed from desktops to completely mobile, as I do not want to sit at a desk and always want everything portable.
I do not trust iCloud (yet) and left all of Apples previous cloud type attempts (iDisk etc.) long time ago.
Dropbox is great and does it for me.
The few desktops I still have I access with Teamviewer when I need it.
All in all as you can see from the many responses the advanced crowd would not buy this and the probably intended target group will not realize until later that 8GB is weak.