I think it would be in Tableau's best interest to allow Tableau Desktop Professional users to run a licensed version of Tableau Server that would allow then to run an instance of Tableau Server that will accept only connections from localhost. This would enable:
1) The ability for a greater number of Desktop analysts to learn how to use Tableau Server, which would increase their understanding and increase interest and demand to get their company to buy a Tableau Server license or Tableau Online subscription.
2) The ability to locally test how dashboards will behave on server before pushing to a shared test or production environment.
3) The ability to locally setup and test data extracts, particularly when recommending changes to the corporate/enterprise data model before publishing to a shared environment.
4) The ability to work on, or test, Tableau Server implementations while offline.
5) The ability to present or demo functionality of the server using a local web browser when not able to connect to a server
6) A broader user base for Server to help evolve the features and UI.
I would recommend that the localhost version also have the ability to have multiple users (provided that they connect from 126.96.36.199). This will also allow testing of various access control grouping and methods without needing to access the server. This important too because the dashboard designers may not have authority to make user/group changes, so this would allow an approach to be tested before it is recommended to the enterprise Tableau Admin.
From a Tableau corporate perspective, I see little downside other than an increase in the number of tech support calls for Server, but this could be either absorbed or have an optional (minimal) support pricing for the localhost users. Another downside is that Server only runs on Windows platforms, so this exact approach would not work for OSX users. Perhaps it could support a VMWARE (or Parallels) installed Server on the local machine limited to the default network that the hypervisor uses for its local (the hardware) network.