I think the most salient application for client-side Python (or Ruby) are for companies already entrenched in server side frameworks like Django and Ruby on Rai

I think the most salient application for client-side Python (or Ruby) are for companies already entrenched in server side frameworks like Django and Ruby on Rails. For all Django provides, developing in it still feels constrained to a server heavy mid-2000's web development paradigm. While frameworks like React and Ember are pushing the envelope on web applications by allowing you to render HTML apps on the server and on the client, to do something similar in Python means duplicating rendering logic between a server-side template language and a front-end framework like Angular, React, or Backbone/Marionette. Some technologies tackle a part of this problem, like pybars, which allows using Handlebars templates in both a python and javascript process. But it would remove a mountain of mental overhead and code duplication, if feasible, to write an entire web application in Python.