I think open source should be the must for any services/apps/development related projects for getting funded by the DAO. DAO is a community fund, community should have some control over the source code too. What if the project developer(s) leave for another chain (steemworld.org could be an example) or stopped developing the app, in these instances the community will have nothing they paid so much for.
But sadly the most expensive project being funded right now by the DAO is a closed source for-profit app.