Although updates to the last SDK version in a timely manner is a valid concern, but does one really rebuild his application with a new major iOS version after a week of the release?
It's often been necessary -- most recently, iPad apps needed to be built against a new version of the SDK to correctly use 2x graphics on the new iPad's high-res screen.
For the major iOS versions, many developers will have an update ready on release day. People will download apps just to play with new features. Keeping current with the SDK is absolutely a business advantage.
You might want to if there's a killer new API feature available, or if there's a new iOS device with a different resolution that requires a rebuild to support.
And the new sdk versions usually coincide with the release of a new device.