Depends which sort of applications you want to develop. New devices have new features, supported by new ios-es. The more applications make use of those features, the more people will buy new devices

Apple encourages to use new features.
From the other side, if developing/testing on a new device, you have a big chance that your application will be unusable/slow/etc. on older devices. This is the reason, why I develop on iPod 2-nd generation!