Because of its faster data-rate, DDR Ram is fast becoming a standard on all systems. To ignore it is to risk leaving a whole lotta powerbooks floating around that require a different type of RAM module to all the other systems around, meaning you'd have to pay a premium to get lower-spec RAM.
Think about it: if you have to buy RAM to suit an older SIMM socket on an older PC, you end up paying a premium because the demand for that product has dropped off. You might even have to hunt around for second-hand RAM modules. By getting in ahead of the wave, Apple are making sure its easier for us to upgrade in the future (especially in a six months, when all new notebooks will ship with DDR-Ram)