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As far as I recall, the saying is "many eyeballs make bugs shallow".
Which ist true. Not every fish in shallow water is caught by the fisher. It is just more likely.
Good coding style and good logical structures which lead to easily replaceable chunks of code can be done in closed-source as in open-source programs. IRIX, AIX and the likes have as clean code as the BSD's and Linux, I am sure about that.
On the other hand, lots of Software companies have to rush out the next release which makes lots of programmers resort to dirty hacks they want to "clean up later".
I one read some study comparing code quality of closed vs. open source software. OSS code quality is bad at the beginning, and if the project continues, the code quality increases. CSS has rather good code quality at the beginning, but it get worse than OSS code quality over time.