They're just trying to kick off PCIe by releasing it before it is really needed. While true that games don't use the full bandwith, newer cards might.
If AGP still dominated (and PCIe wasn't endorsed like it is now) then cards/games that really utilize the bandwith would have to launch with PCIe, but then almost all gamers would have AGP. I believe they released PCIe now, to establish it in the market, so that when newer cards/games could use or need higher bandwith, PCIe wouldn't be this brand new technology that everybody would have to upgrade to for a new card.
They're just planting the seeds for PCIe so by the time they need to pick it, it would be grown enough to do so. They realized that AGP would still hold it's grasp in the industry, and they would need to slowly eat at it with PCIe untill PCIe is common throughout consumers.