Having been intimitately involved in the bringing the pattern library live I can tell you first hand the simple reasons they have gone public. And these are good reasons for others to do the same.
- Goodwill. Sharing back with the community always pays back in rich dividends.
- Feedback. Lots of eyes and lots of hands make the libraries better. We recognize that there are a lot of smart folks outside the halls of Yahoo!
- Dialog. Brings issues to the forefront for discussion. Like accessibility for design; common design libraries and language.
- Reuse. Best corporate reuse strategy is open source. I saw it at Sabre with Rico. Same at Yahoo. Already groups are using the libraries more now that they are public. And it energizes people to participate in something bigger than all of us.
- Standards. If some of our better design ideas spread then it is good for our users and in turn good for us. Relearning different ways to do the same thing is only fun for so long :-)
- Mindset. Yahoo! has a lot of folks in management, design & engineering that have benefited greatly from the open source community. This gets in your blood. It becomes part of your value system. So it becomes a natural reflex to do it this way.
And yes, we don't share everything. We have lots of really awesome research going on. Really cool stuff coming. Sorry that won't be in the next release of the libraries ;-) But I think that is understandable.