actually, xbe's do have media flags embedded in them so that they will only run from certain media and if the region flag in the xbe matches the region of the xbox. it is documented in the xdk that there are separate media flags for hard-drive, dvdrom, xboxdisc, and others, but there is no documented discernment between dvd+r and dvd-r.
ultimately, it IS the RSA key that prevents backups from running on an unmodified xbox. if you edit the xbe's to change their media flags, RSA key no longer matches the hash of the xbe. i've read seemingly ambiguous information regarding whether or not the actual disc is signed, so i'll take someone else's word on that. the bottomline that everyone should accept tho is that a backup will never run on an unmodified xbox. unless someone gets REALLY lucky, it will take YEARS to crack the MS RSA encryption key.
no information such as music or cut-scenes are lost when backing-up an xbox game..unless you purposely leave it out to save space.
a modchip is not necessary to modify an xbox. you can flash the tsop (onboard bios chip in xbox) or you can use an audio or font exploit. the UDE exploit is really living up to it's name. if i were just starting out, that is the route i would take, but tsop flashing isn't very hard either and it makes lots of things easier and less complicated...but i digress...