Detail comes from the original picture. With zoom feature, a higher resolution display isn't really necessary. Higher resolution doesn't really resolve the issue that the object is small. For example, iPad still cannot compare to a 12-15 inches X-Ray film.
These Apple products (iPad3, iPhone4S) are the bacon of the telecom space - everyone likes bacon even though it clogs your arteries (read telecom pipelines).
A related article below calculates the new iPad3 can potentially consume your entire month's bandwith cap in about 10 minutes depending on what you are down loading. Maybe they exaggerate, maybe it might take a full day?
The Siri feature on the iPhone 4S consumes 3x more bandwidth than an already piggy iPhone 4 without Siri.
As these new Apple-bacon (@copywrite) products proliferate, the bandwith pipes and towers are going to get mighty clogged. AT&T and the other carriers have no desire to sink dollars into improved infrastructure without payback. Collision ahead. This reminds me of the days when Microsoft first began writing "fat-ware", as opposed to software which was efficiently written code. That fatware demanded faster and faster Intel processors (and cost) to pump all that fat code around.
The higher pixel density will be a boon in certain fields such as medicine, whereby the finer details of X-rays and radiographs will be visible instead of blurred or completely missing.
By the way, get your eyes checked!