And there are over 400 million iOS devices. If you think you can get a large enough percentage of iPad 1 users to make it worth you while then develop for it. I'd much rather spend my time ensuring it works well on 300+ million other devices.
There are certainly fewer bombshells in iOS6 than iOS 5, but it's no stretch to say that adopting iOS 5 early on was worth 100s of hours -- it had huge improvements for dev.
Also keep in mind that the fewer OS versions you target the less testing you have to do.