You can get exactly the same benefit by blocking non-established/non-related connections on your firewall. NAT does nothing to help security.

Edit: BTW--every time I see this response of "NAT can prevent external access", I severely question the poster's networking knowledge. Like to the level where I wonder how you manage to config a home router correctly. Or maybe it's the way home routers present the interface that leads people to believe the two functions are intertwined when they aren't.