Until recently, I have owned a 11" Macbook air from 2011. Just 2GB of ram but 256GB ssd. It was fine for travelling, and it did what I wanted it to do on those travels. I justified it to myself, that I don't need a "powerful" Macbook of any kind when I have quite powerful Mac Pro where I'm willing to invest even more in order to make it more powerful
However, I have owned a 2012 13" version with i5, 8GB ram and 256GB SSD and also a 2013 model with same specs. The 2013 model just flied compared to the 2012 it was amazing, and I bet anyone using it right shouldn't have any smart reason to upgrade to 2014 or even 2015 model unless they don't know what to do with their money.
It was also mentioned, that the 2012 model would see only 3 more years of support of something. I would actually bet on 4-5 years, since there are even older mac models out there which still support installing Yosemite on them, such as the 2007 iMac, Early 2008 Macbook Pros and even the late 2008 Macbook air