It's a question of "must have" vs "nice to have" IMO. If you feel your setup is hampering your productivity than it's an easier question to answer.
In my case, I am upgrading from a 2013 MBA to the base 13" tbMBP mainly because I wanted a Retina display (finally) and my wife would be upgrading from her heavier 2010 MBP to my MBA.
Laptops have useful lifespans of 5-7 years if you can replace their batteries and take good care of the equipment. Usually operating systems will drop any given model after so many years but you can still run for another 1-2 years on a stable OS (like you can on iOS devices as well).
I am expecting a big boost from much faster storage and to be amazed to finally have all retina displays in my workflow. The upgrade CPU will be hardly useful and my wifi speeds were already very good (802.11ac).