I have learned a lot from reading Open Source code, and so have most of my generation programmers, and the generations following us.
There are many reasons for releasing something as Open Source - my motivation is a perspective where I view code as an art form - and as an artist I want to share my creations with other artists (programmers). That's also the reason War FTP Daemon version 1.* never was released as Open Source - the code is "commercial standard", but not art. In my experience, Open Source code is of way better quality than traditional, commercial code (well - at least the mature Open Source projects).
There exist a number of Open Source software licenses, where Gnu Public License (GPL) is maybe the best known, and the one that best protects the rights of the software consumer - the end user. GPL was not written only to protect the software authors or artists - but most of all the end-users.
Microsoft's recent attack on GPL is a serious threat to all software users, as it aims to take your freedom away. You should follow this attack carefully and read this important article. Please feel free to send a letter to your favorite computer magazine, stating your opinion on this.