Hi Ryan,
Thank you for your suggestion!
Checksum verification is required when you download file from third party sites/mirrors, you then take hash from original site (where download link is posted), download a file from the mirror, and then verify that the file is not tampered. In case you download from parallels.com, it is not required, we don't have mirrors as Parallels pays for CDN service to ensure your trouble-free and fast download. Transport protocol cares about accidental corruption while downloading.
Your concern on having only genuine software and providing admin password to make install is absolutely valid. I would like to emphasise the work that Apple does to protect its users from tempered downloads and malware. With GateKeeper and necessity to sign application with Developer ID, it is not that easy to install software from pirates and hackers. And it seems that with Apple Notarization service that was introduced last year your security will further improve .