That's exactly why it's valuable to you as a reference for a future employer. "I built this whole thing" is way more impressive than "I helped align the dropdown menu on this one FireFox options window".

As to giving back, there's nothing stopping you. My latest for-profit venture has spun off two open source projects that "the community" can use in the future. Past projects have done the same.

The key is to do things to benefit yourself, with open source being a side product. That's how big companies do it. Why shouldn't it be the same for us little guys?