I accept this is the perception many businesses have - but it is generally fallacious. Taking your example of the forum software - do you really think it is the bells and whistles of the forums that draws people? I doubt it - I am sure you will find it is the content - so a competitor who ended up with any enhancements to an O/S forum solution would still need to get the content right to benefit. And most of the other users of the forum software would not be in competition anyway. Now there will be exceptions - I know of some examples of software that is truly unique to a business so would never be released as open source - but this sort of software is almost inevitably not bought in from outside anyway and so the open source solution would never be on the table.

The original topic was comparing the pros/cons of bought in versus self built solutions. If bought in was ever an option then you could not have been relying on customisation for competitive advantage anyway as the bought in software would be available to all you competitors.