The new iPad has a quad core graphics processor. Does a tablet really need more that? Usually multi-threaded applications perform tasks such as rendering, encoding, or performing complex calculations - not the sort of thing you would typically do on a tablet. The iPad's A4 was clocked at 1 GHz, the iPad 2's A5 at 850 - 900 MHz (it fluctuates with usage). While clock speed isn't a direct indicator of performance, there's certainly headroom for improvement in that area without needing to add an additional pair of battery sucking processor cores.
What are the types of applications taxing the A5 CPU? Certainly not consuming media, video conferencing, note taking. It's games! The portable gaming market has seen a huge paradigm shift. As Nintendo DS and Playstation Portable sales decline, iOS devices have tightened their grip on the market share and wish lists. Cartridges, discs, and long story centered games are being abandoned in favor of short, clever downloadable iOS gamelets. Why buy a $50 portable title when you could have 50 little games to feed your attention deficit? Each year iOS devices lunge further and further towards console quality gaming- both in terms of polygon pushing power and game quality. More cores equals more game quality potential. With the potential advent of an Apple television and the increasing pervasiveness of cheap AppleTV boxes, it would make sense for Apple to beef up the engine in their cash cow into an all-in-one portable powerhouse that can stream gorgeous gaming graphics to your large TV. While an additional graphics cores will have little to no impact on office applications or media playback, Apple's gaming future is golden.