This kind of thing is such a personal matter. My mother, for example, usually has a couple of Safari tabs and maybe a Scrivener project open. She wouldn't notice if I upgraded her RAM from 4GB to 8 (not that that's possible on a MBA but anyway...), and I doubt if she'd notice if I somehow cut it to 2. But me... I'm a University student who uses my MBA as my main machine, for browsing, research, writing... and although people always seem to say 'oh if you're only doing that 4GB is plenty', but for me that hasn't proved to be the case at all.
For example: for my last essay I had 4 Scrivener projects, 5 Text Edit files, 4 Finder windows, 20+ PDF files, and over 80-100 Safari tabs, 10 + images, plus all the background apps and processes I use, and my page-outs were more than my RAM. And I DID notice. SSD or not, I was getting slow-downs constantly, and even a few crashes. 8GB of RAM is a *very* tempting upgrade for me on the 2012, and I will at least seriously consider upgrading my 2011 machine for the new one. The other upgrades, the USB3 and the faster SSD make it more tempting, but it's the RAM that really matters.
So I guess all my rambling really boils down to: no for some people it wouldn't be worth the upgrade, but they probably wouldn't be considering it too much anyway if their usage is *that* limited. But for many it *is* worth it, either now or because they can see their usage scaling. And IMO it's far better to drop an extra $100 now and find you didn't really *need* it 3 years later than it is to buy 4GB and find in six months that you need 8GB and can't upgrade it. And sometimes it's not just the tasks you do, but how many of them you do at once!