Each to their own,
iPad 1: First iPad, breakthrough product, good buy
iPad 2: Incremental update, bad buy
iPad 3: First iPad to have Retina screen, good buy
iPad 4: Not compelling enough if you have the 3, bad buy
iPad Air: Substantially lighter and thinner, good buy
iPad Air 2: ??
This thread is about longevity.
The iPad 2 has the same/similar specs to the iPad mini which is still being sold in 2014, so chances are good it'll be getting updates for years.
That iPad 4 gives you continuity which extends its usefulness many times over the 3.
If the Air 2 has 2GB of RAM it will also be getting updates long after the Air has been abandoned...