There's some advantage with not having physical buttons.

Most notably you can quickly "cancel" an intended action with a non-physical button by sliding your finger away from it while holding down.

e.g. you're playing a game and accidentally press the home button. slide away to continue playing. You can't do that with a physical button unless they implement something where holding the button down causes a less intrusive action. iOS does this i believe by not exiting the app but bringing up the multitasking menu when you hold down the home button.

I currently have a thinkpad tablet with physical buttons, and I'm afraid of using them half the time because of accidentally hitting them.

I don't understand how a gestural interface would become a guessing game any more than physical buttons would be. Regardless whether your interface be gestural or physical certain actions would presumably do different things depending on the current context. Instead of remembering to press a particular button, one would remember a particular gesture.

Unless you are talking about guessing whether your gesture was recognized.