Most smart phones today have the option for you to password lock your phone. Meaning every time you turn on or wake the device from sleep, you'll have to enter password to gain access. Believe it or not, some people do use this feature, despite the inconvenience. What I'm suggesting would be no different, except you'll have the option to easily switch to guest account with restricted access.
I agree locking individual apps would be fine if it's something you don't need all the time. But if it is something like emails/txt/photos which I access many times through out a day and many times with in the same session of using then yes i think it would be a more of a pain than just logging in when you first access the phone.