With the huge requirements of the Cell processor come bigger requirements in the media on which games will need to be stored. This is why, according to Maguire, the Blu-ray discs are a must-have for next-gen games. Thanks to the 50GB of storage, people won't be having to change discs in the middle of loading (which is apparently what we'd have to do he says). While this is spot-on for more content, it means that developers are going to need to deliver on that proclamation for quantity and quality.
By adopting this high definition medium, they hope to "legitimize" the HD era where it counts most: in your home.