The problem with free external security audits...
...Is that someone ends up paying for them.
Usually, the auditor in question is employed by a company that is trying to sell you something. Guess what said auditor's recommendations will entail...
Whilst your point about no-one being a master of all skills is well made, it is important to get an objective and unbiased view from any audit. I've yet to see that from a free audit.
Vic.