Well, the old downloader merged the file, performed integrity check, and then, if something was wrong, it downloaded corrupted chunks. (installer was downloaded in chunks). If everything was ok, downloader deleted all chunk files, leaving only merged file.
Many people have internet connection with monthly bandwidth limit, so it was possible to save it, with old downloader. with big games like witcher , painkiller , redownloading the whole file from the scratch is a real pain in the ass.
In that case, new downloader is even a downgrade, compared to the old one...
This is the same case as well. The games are downloaded in chunks, which are verified with md5 when completed and re-downloaded if something is wrong. When every chunk is downloaded and verified, the game files are merged, and chunks removed.
I'm talking about a situation, when you have only completed installers on disk, with no chunks to verify. In this situation I can only re-download the whole installer.