The HD-DVD drive is only opintal and no, Microsoft has even said that they will NOT make games for the HD-DVD drive...only for the HD-DVD movies.
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Xbox.com | Xbox 360 - Why HD DVD?
Why Add an HD DVD Drive?
With Xbox 360, you have the ultimate freedom to create your own high-definition entertainment experiences. By offering the Xbox 360 HD DVD Player, Microsoft is providing yet another option to enjoy HD content, with the understanding that next-gen DVD is not for everyone. Microsoft's primary emphasis remains creating incredible HD gaming experiences, but they also provide a host of options to take advantage of the powerful technology in the Xbox 360 to enjoy HD photos and movies, if that’s your choice.
They never said anything about the games...
Why HD DVD?
It's no secret that there are two next-gen DVD formats hitting the market. The first to market was HD DVD and the second (indeed already out in Japan) is Blu-ray. After two years of deliberation, Microsoft made its choice and decided to back HD DVD. The question then becomes why? Here are a few reasons:
Manufacturing: Concerns developed regarding Blu-ray's ability to manufacturer the discs at the capacity originally intended. In fact, with the Blu-ray launch in Japan, the discs are being created at single-layer 25 Gigabyte capacity, which is half of the originally intended 50 gigabytes.
Easy to Damage: Because Blu-ray discs are encoded near the very top layer of the disc with limited protection, the data is literally at higher risk to damage. Conversely, HD DVD maintains physical protection similar to standard DVD.
Copy Protection: Blu-ray utilizes not one, but two different forms of copy protection, adding an extra layer of protection and complexity that just didn't appear necessary.
VC1 Compression: While both Blu ray and HD DVD players are required to be compatible with three different compression technologies (including VC1), an intriguing trend has already begun. Content providers (movie studios) for HD DVD are nearly unanimously using the brand new VC1 compression technology, whereas Blu-ray providers are sticking with the ten-year-old MPEG 2 compression that has been used on standard DVDs. With VC1, HD DVDs are able to compress much higher quality video into a significantly smaller package.
Capacity: Dual layer 30 GB HD DVDs are already available, whereas Blu-ray has only released its single layer, 25 GB disc.
Hybrid Disc: HD DVD offers the ability to encode both a regular DVD format and HD DVD format on the same disc. If you purchase a hybrid disc, you can flip to one side and play the movie on any standard DVD Player, and when you do get an HD DVD player you can flip to the other side and play it in true HD.
Whats even more interesting is that if you don't have a HD DVD player and you want to purchase a HD-DVD format, it has two sides one is non-HD while the other side is HD for use best with the HD-DVD player.