The question should be, Why should Apple make all their products 'iThis' or 'MacThat'? Stepping away from the 'iPrefix' may not be so bad; iVariety.
This is an excellent observation. With the saturation of me-too "i"-this and "i"-that products, there exists the potential for the branding to become meaningless and easily confused.
Today, every facial tissue is called "Kleenex" by the public. This is good and bad. When someone thinks of Kleenex, there's only a fraction of a chance that they're going to buy that actual brand at the store. They may buy any number of inferior brands.
This type of association is great for products that are 2nd or 3rd in the marketplace, but never good for products that dominate since there's nowhere for that association to pull them but down.
Apple shouldn't want their products to be lost in the sea of "i"-everythings... it's time to move on and in a way, the

branding is a means of differentiation that no one else can copy. Not only is the logo convenient for truncating the name for easy mention in advertising and other mentions, but it's an extremely smart protection on Apple's part. There are many ways a company can call a product "Apple" something since the word itself is not trademarkable...
However, the bitten-Apple logo IS a trademark, and no product can copy it legally. This type of branding strategy ensures that their product will be remembered as a uniquely Apple, Inc. product and guarantees that no me-too products will be able to dilute the perceived brand value.
Last, the

logo reinforces in every mention on an ad, on a blog, in a news piece, etc. the identity of the company that brings these technologies to the consumer. Sure, it's inconsistent with iPod and iMac, but if Madonna's career in the 80's is any proof of the power of constantly re-inventing one's identity, then it should be evident that Apple, whose product introductions keep setting competitors behind 5 or more years, is taking a direction to keep five steps ahead in the branding game. Given that Apple's name is readily identifiable as the single most admired brand across all product categories and is identified by the general public as synonymous with quality and ease of use, differentiating their corporate brand identity is a very critical component in their ability to command the kind of margins they do and continue to be extremely profitable despite economies of scale.