• Benefit: you are legally guaranteed that people make their changes/contributions available to you.
• Cost: many commerical users can not use your code. They will not use your code and thus will never contribute.

Even the LGPL can be problematic when they need to statically link the library.

• Benefit: you are legally guaranteed that people make their changes/contributions available to you.
• Cost: many commerical users can not use your code. They will not use your code and thus will never contribute. See this thread explaining why the libcinder people can not use (L)GPL code. Even the LGPL can be problematic when they need to statically link the library.