I have 8 GB of memory on my 2013 13" MBook Air - I run 23 separate applications simultaneously, including the full office Suite (Outlook/PowerPoint/Office/Excel), VMware Fusion w/Windows XP + OpenBSD running, Dynamips Routing Simulator w/10 Cisco 7200 routers, Google Earth, Aperture, Pixelmator, etc.... No swapping. Everything runs fine.
OS X (in my case, Mountain Lion, so an old version) - does really, really well with shared libraries. 95%+ of consumer users (not pros, whose needs are obviously more rarified, and for whom the sky is the limit) are probably fine with 8GB of memory, and it's not something I would recommend them increasing unless they plan to hold onto the computer for more than 5 years.
OS X (in my case, Mountain Lion, so an old version) - does really, really well with shared libraries. 95%+ of consumer users (not pros, whose needs are obviously more rarified, and for whom the sky is the limit) are probably fine with 8GB of memory, and it's not something I would recommend them increasing unless they plan to hold onto the computer for more than 5 years.