The 'last snapshot as a splash screen' actually traces back to Jeff Raskin's work on the Canon Cat. It counts on the user trying to remember where he left off

The 'last snapshot as a splash screen' actually traces back to Jeff Raskin's work on the Canon Cat. It counts on the user trying to remember where he left off while the program loads, so by the time you think "aha, I was gonna send an e-mail to Ron" the UI is responsive. It's a nice trick that saves you time in the end. Of course, if the app takes too long to launch the UI would seem unresponsive, which is why Apple chose to not make it available to third party developers.