If you're always with groups of other people — friends, family, co-workers, etc. — or if you travel often, it makes sense. As I often do with my MBP, when I'm out of my office to meet with clients or vendors, I'll set the startup to require my password. But the phone, always being in my pocket and never left unattended, is less of a risk. That said, it's easy enough to enable the passcode protection if I'm gonna be in an environment or situation where the security is well-advised.
_________________________