Your smart phone is likely the portal to a lot of your personal and professional life.
If you can get into my phone, you may get access to my e-mail, my Google Drive, Evernote and more. You'd also have access to Google Authenticator, which generates one time passwords for use with two factor authentication.
My firm has a very sensible policy that dictates all mobile devices attached to the network have to be password protected. But when it comes to information security, I'm a bit of a belt and suspenders kind of guy. I heard about an app called Perfect App Protector Pro in the Google Play Store which allows you to add an additional layer of security for selected applications. You can tell it to require a PIN before a user can access any apps you select. So, for example, you can't get into my Google Drive, Dropbox, Evernote, Authenticator, Gallery or my phone settings without entering a PIN that's different from the one used to get into the phone in the first place. You can also set it to require a PIN to make a phone call or to connect to an external device by USB.
I'm sure it's not foolproof, but it's better than nothing.
If you like your belt and your suspenders, and want to keep your mobile data locked down, check it out.