Most pay per download programs are really pay per install, which many are confused about or should I say surprised! This happens because while the advertiser or publisher sees the start download, some do not finish the download process(Install) and therefore do not receive commission. However, bundling is the new wave taking over the industry, by using an .exe file one can bundle it's software within a download installer of games along with other endless free software downloads. I was at Ad:tech San Francisco a couple months ago, and was surprised to see ALL of the ad networks were only pushing CPC and CPM models, not realizing the power of money being made through bundling software.
thats because any one with money can start with CPC and CPM advertising while not everyone has the software (source code) to do co-bundles. Even if you have your own source code, you still need to find ways to gain installs.