I smell some false dichotomy here. I don't know much about iOS development, but it sounds baffling to hear that iOS 6 is so different that it would take "hundred of hours" just to make sure the app is usable on the previous release.
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.