I think there is another issue to remember, you can't trust the source. So, I doubt any security professionals would say its preferable to not have the source,

I think there is another issue to remember, you can't trust the source. So, I doubt any security professionals would say its preferable to not have the source, but to do the job of verifying the software you have to do it at a lower level than the source anyway, so having the source is a bonus, not a requirement.