Even some of our major tools (like Beaker, the product I am development lead for) are open source rather than proprietary, simply because we think that collaborating in the open is a better way to build software, even if all the developers on that project currently work for the same company.
Defaulting to open is about creating opportunities for serendipity - assuming nobody else would be interested in our software, and hence keeping it a secret would be a self-fulfilling prophecy. Assuming competitors would be interested and being overly restrictive on what we release could also be counterproductive (and I think we're actually undersharing in several respects at the moment - I've opened up some aspects more since taking over as dev lead, but we still have a lot more to do before Beaker is a truly open project that clearly explains its potential value to users and provides good avenues for them to progress to becoming contributors).