Why have a full 5.1 Dolby/DTS decoder etc on the CD/DVD drive? For an average user they are not going to have the 5.1 speaker set or need the decoder even if they do.
If they DO have a 5.1 spaker set, the higher up ones (like the Logitech Z-680's) have a hardware decoder built in.
You can also have a soundcard with the decoding built in.
With any 5.1 soundcard (like Turtle Beach Santa Cruz/Videologic sonicfury/xplosion, Live!5.1 and up, Herc Fort etc) you can get software decoders like in PowerDVD and WinDVD to do the decoding for you.
Thats four levels of Decoding already - will people actually pay for it on the drive, if in fact they either don't need it because not enough speakers, or don't need it because they have 3 other ways to decode the audio alrday?