For most people who use their iPad for a bit of fun like web browsing, watching a few videos and accessing email, I really don't see the need for upgrading. To me, that's what an iPAd is - an entertainment device, not something to work on; I have the laptop for that. I have iPad 2 and I can't see myself buying another iPad until the 9th or 10th version comes out by which time I would imagine the iPad will be very light, have very long battery life, very high quality screen, noticeably faster processor. But I really don't see why people buy a new iPad each time a new version comes out. Most of the upgrades are very small and incremental. Apple deliberately holds back big changes because they need to advertise something new each time they release a new one.
For example, if Apple brought a new iPad on to the market with only one change, let's say, a superior camera, there will be a flock going off and buying the new one even though most will never use the camera. Who uses an iPAd to take pictures? Its so awkward. Lot of the new iPad purchases are irrational.