The latest fetish seems to be open source. It will pass. It comes from reddit culture and general praise-seeking behavior online. Contributing to open source == upgoats.
Security? Auditing the binary is the only way even if you have the source code. You can't be an expert in every language, so it's easier to read disasm.
Modification? Easier to just find another program that does it, use the program's integrated scripting env if it supports one, or write a program yourself for that specific purpose, which will be quicker than trying to get up to speed on a whole big project.
The money? The deficiencies of open source programs compared to ones people are paid to maintain justify whatever the program may cost. Most closed source programs don't cost anything anyway.
Guess the only legitimate reasons to use open source is that you're just gullible, clinically paranoid, NEET, trying to fit in among other internet idiots who use le cool shitty OSS, or a non-programmer.