My favourite development machine is still my 9 year old Air with 4GB. I mostly do frontend, a bit of Rails and some of GameDev with C++. It forces me to use lightweight tools (like Sublime) and it works like a charm.

One advantage of having it is that the company I work for has a very international and diverse user base, but gives us developers super modern machines. Some developers tend to overgineer and cause performance issues on slower computers. With an older laptop I can understand better how the app will be perceived by users.