Siri really isn't needed, the dictations is a great addition though. Being able to just say a note, or an email will be great. And since it goes through apples servers, it will be just as accurate as the 4s
Reducing server load. The 4S is probably stressing Apple's servers, and Apple may not have sufficient capacity to handle the horde of iPad users.
Keeping Siri as a "big new feature" for a future iPad. Apple needs "big features" for each product release, and Siri would be a great addition to the iPad (because it potentially makes a great personal assistant, regardless of what the "it-only-makes-sense-on-an-iPhone" people claim). Today, the big new feature is the retina display. While announcing Siri for the iPad would have been nice, Siri can now be used as a "big feature" for some future iPad. (Admittedly, the one big argument against this is that Apple wants to keep this as an iPhone-only feature, not because it only makes sense on an iPhone, but to entice people into buying the iPhone.)