I do a lot of frontend development and most of the time I am using 2 external screens + my mbp. On my mbp I have my terminal open, so I can see linting / build errors. On my primary screen I have sublime + browser and on my 2nd smaller screen I have sketch/photoshop open.
If you have to get values from ps or sketch files, get margins etc. it's awesome to have two screens.
However, if I don't have strict designs that need to be pixel perfect or I do more development then styling, I also enjoy doing everything on my mackbook screen.