With open source the file format is always documented, at least in the code itself.
As great as that is, I believe it's a bad idea to use this as a selling point for OSS. I mean, the theory is great - everything is open, all the information you could ever want is documented in some way and if you happen to find a bug or whatever, you can go in and fix it yourself.
But what if you're not a programmer? What if you're just an average Joe who knows an average amount about computers? (I.e. not a lot short of turning it off and on and maybe running the odd Virus scan).
A car is open, if you're a Mechanic and something goes wrong with it, you can just open it up and replace or fix whatever is broken - but of the millions of car drivers out there, how many know how to do more than change the odd flat tire?
I think if you presented OSS in this way, the average person is more than likely going to get scared off by the prospect of having to be a programmer just to write a letter or whatever.
Sure your not a programmer, thats fine. The advantage of having the source code applies to all users of the software. For example, if it is important to you pay a programmer to fix it for you. This is no different than a support contract with a appropriator software vendor. The advantage of having the source code thought is that *any* programmer that can code in the language that the software is written in, can write the fix for you.
This is why a smart company that is having another company build them a core system will have the contractor put the code of the system is escrow in case the contractor gets hit by a bus, fails financially, etc. The other company then gets the code from escrow and hires another company to continue building the system.
To go back to your car example, okay so you are not a mechanic and so the open specifications of the car are useless to you personally (as in you can't fix it yourself), but as with my previous example, you can hire any mechanic that is qualified to work on your car and understands the specifications of the car can fix it for you.
Don't forget about opportunity costs. While some people are able to use what the specifications of the stuff they buy or the source code of their software that they use. It may cost them less to have someone else fix their problem for them, while they are doing their higher paying normal jobs.