Other than obvious value in promoting your apps for free, the answer is in understanding the concept of "fill rate". Ideally, for every ad request you make to your ad network it would return an ad that you would be compensated for (at least potentially).
Unfortunately, we do not live in the ideal world and that is not the case. Fill rate is a measure of how many ad requests are actually fulfilled with an ad. And it's almost never a 100% (the ideal case). According to various reports fill rates could be as low as 10% on average across the mobile ad networks worldwide. So, unless you are complementing your primary ad network with other ad networks and/or "house ads", you are wasting the valuable ads space quite often. Maybe even 9 times out of 10!
AdDuplex is a great way to complement your commercial ad network(s) as a fallback solution for cases when they don't have an ad to show. Due to its nature AdDuplex has a virtually 100% fill rate (the only case when you aren't getting an ad is if there are some connectivity issues). This way you can combine the best of both worlds: get paid when commercial networks have the ads to show and get free advertising for your apps when they don't.