Well, this is just MHO, of course, but I would think that building the ac spec into an iPad would instantly make it a mainstream consumer thing, something the average consumer would want to take advantage of without feeling they're being gouged.
It may have been somewhat exotic up until now, confined to a few devices, but build it into the iPad and now you've made it a standard. I don't think the public is ready to pay extra for something that should be standard. I also don't think the average public would know the difference between the specs, as NMF said, but it will surely be used as a sales tool (Wireless twice as fast as the "old" iPad"!!!)
But then, as (s)he also said, this is Apple.