"Given enough eyeballs, all bugs are shallow."-Linus' Law
Open source code is transparent. Anyone who knows what they are looking for can examine it. The community of users are volunteers testing applications that are properly tagged as alpha or beta. That community benefits from free software. Software that can be modified without legal impediments. 
When I first encountered it, my skepticism is like how can anything work? Since joining a community (even without programming skills) I have seen things work from the inside. It's amazing that I give so little time (free time) and get an operating system and all the software that I need to do my job. I also get excellent support when I meet challenges.
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