"Rule #30: It is mainly the incompetent that don't like to show off their work."
Madden's remark is based on the specific environment he was involved in (NASA engineering); if you're surrounded by people who are good at their jobs and who know how to criticise constructively then there aren't many reasons to hide what you're doing other than it being a bit rubbish. I don't entirely agree with the idea that people who don't show things off are necessarily incompetent, especially if showing off your work means putting it under general public scrutiny (who can be very nasty), but there's a huge amount of value to be gained in getting feedback from your peers as early as possible.
[1] http://www.nasa.gov/pdf/293253main_62682main_jerry_madden_fo... (Non-PDF version: http://www.altisinc.com/resources/rules/ )