A friend recently put up this question on the board. He comes from the Microsoft world and is one of the developers at a startup right now. His curiosity fetched him Erlang and Node.js for servers, ember.js and backbone.js along with RabbitMQ and Celery and so on. All these are open sourced, and hence this question.
His guess was marketing. I cannot deny that today the open source adjective gets you some visibility. That cannot be the primary motive though. This visibility brings a lot of pokes, bugs, requests, pulls and forks along with it. They are a very good source of headache if you are not expecting them. My guess is that the open source world is a dream come true for authors who are looking for criticism, suggestions, or comparison with the competition. It lowers the barrier for adoption to the lowest possible level and encourages users to throw bouquets and brickbats at the author. If managed properly, this is an effective way of evolving a tool quickly, faster, or at least at the same pace, as its competitors.
What do you think?