[IgorPartola]

Sure. Your grateful users might add a feature you didn't have time for. Or they might find a security issue and send you a patch as a part of responsible disclosure. Or they might start hacking on your codebase and actually do something cool. Then you can hire them/buy them out, turning your codebase into a nice recruiting tool.

Think about this: if Twitter open sources 100% of their code, what would change? Would Twitter clones pop up all over the place? No, because Twitter is the software + infrastructure + name. Most companies think that their code contains some type of secret sauce that makes it special. In reality that's not true. Sure, Google may hide their exact PageRank algorithm, but they don't need to hide their web server code. Or their indexing algorithm. Companies like Twitter are even better for this: they do absolutely nothing that's really proprietary.

For an example of this on a much smaller scale look at TheTVDB (http://thetvdb.com/), their entire site source is OSS, yet there are no clones.

[d23]

For some companies where the barrier to entry is very high or requires a lot of market share to compete, this may be true. But for others where the investment in actually building the thing was the only barrier, giving the code out to the world can be a terrifying prospect.

[IgorPartola]

You are right, it's not always appropriate. However, in most cases, it's just fine. Here's a list of companies that I think could open up at least 95% of their code: Netflix, Hulu, Amazon, Tesla/Ford/GMC/Toyota/BMW/any car manufacturer, Apple, Home Depot/Walmart/Safeway/any retailer, Mint, Twitter, Facebook, Google, and many more.