Personally, I think the idea of a download manager (or even just allowing use of certain existing download managers) would be much more workable than PARs. The Files screen is already pretty cluttered, especially for those of us who purchase prodigiously.
Don't some download managers use some kind of checksum, checked against the server?
What download managers can't be used with RiffTrax now?? In the early days you couldn't use them because of the 3 connection limit but that has been gone for quite awhile now. Do download managers prevent corrupted files? The beauty of par files is if you download a 1 gig rar part and it is corrupt you then only have to download a 20-100mb file to repair it. Instead of having to download the whole 1 gig again.