Apple doesn't want the ability to plug in more storage because then they wouldn't be able to come out with the new 128 gig version, or the new 64 gig version before that, or the new 196 gig version in a few months.. Or the new bigger battery version, or the new every few months at a massive price premium.
Things like more memory didn't use to warrant a new version, customers could just buy more memory and stick it in themselves. Not with apple, more memory means buying the new $800 version of the same thing you have, instead of a $20 memory chip...
It goes against apples very business model. Their business model is marketing, they know how to work their target audience. They know how to extract lots of dollars from a majority of tech illiterate people(not everyone is, but the majority of apple customers are). Their core business is marketing, not hardware/software, and they do it very well.