As far as App Locker is concerned, I think app developers should make password protected apps. But, MS should think of protecting messages by providing some inbuilt lock/hide features.
I hope people don't defend MS here
Let me give you a scenario, in an enterprise environment (now I say enterprise because I know the content on my work machine is something I fear of sharing or is plain prohibited to share with others), when I unlock my machine I use network login credentials to by pass the windows login. Imagine if I have to open Visual Studio for development it asks me for a password, or Outlook to check my emails it asks me for password, or heck My Computer (explorer) it asks me for password again! (see what I am trying to get to?) User experience goes kaput if this is functionality even exists.
Now if you trusted your friends or the people you share your phone with, you wouldn't be asking this question here. If there is something you want to show you can always add it to the kids corner and it works well, swipe lock screen, enter password and give it to the person to check whatever he/she wants to check with access limited to only that application.?
And about defending MS, no one should and even if they do its their opinion. This is a discussion thread so every ones opinion should be respected including every one posting here.