However, as much as technology has advanced today, there is one thing that will never be replaced: YOU, the photographer.
The camera will never be able to pick the angle, or go astray of the settings to get a properly exposed picture in order to achieve an artistic shot, or when to shoot at all. Not on its own.
If you rely heavily on AF, its possible that youve come across the following situation:
Youre taking pictures outdoors, you see a squirrel jumping from tree to tree and suddenly starts dancing in a funny way. You half press the shutter (as you should always) and try to get the focus lock to get pictures of the squirrel dancing, BUT, the camera can NOT achieve focus lock and locks the shutter. The squirrel hears you cursing the camera and runs away.
And the moment you wanted to capture its gone.
Sometimes, the moment itself is more important than razor-blade sharp focus, intense saturation, focal length used or worrying about if the ISO youre using will yield too much noise. Sometimes you just have limited seconds of something incredible/funny/odd/rare/etc. happening or someone doing something incredible/funny/odd/rare/etc. and you dont have much time but to look at the frame and shoot.
Sometimes, the camera wont fire simply because it didnt achieve perfect focus according to itself, the camera wont care if what you tried to shoot will never happen again on your lifetime or ever again.
But you care, you are the photographer, you are in COMMAND!
In order to prevent missing those events (or even events you can shoot later), there is an option you can use to your advantage in Sony Alpha DSLRs: Shutter Release Priority.
If you use an Alpha 200/300/350:
Menu-> Recording (camera icon) Menu 2 -> Priority Setup-> Release
If you use an Alpha 700/900:
Menu-> Recording (camera icon) Menu 3-> Priority Setup-> Release
The default setting for the camera is AF, in this option the camera wont fire unless it achieved focus lock, but if you want it to shoot when YOU tell it to, pick Release.
In theory, I think the shutter lock function was included in the camera in order to PREVENT the camera from taking pictures with a bad focus. Part of the automatization in the cameras requires for them to do the job perfectly ALWAYS. It also acts as a fail-safe mechanism that guarantees you will get perfect focus on every shot (as long as the camera has achieved focus lock).
This is a good thing if you plan the camera to do everything for you and you only want to worry about picking a subject to shoot at. But the downside of this approach is that you are leaving the camera with 95% of control regarding how the shot is taken, and if you come across that squirrel, you wont be able to fire if there is no AF lock.
If you still want to let the camera worry about most of the settings but you still want the camera to fire when you want it to, set the priority to Release.
Keep in mind that setting the camera to Release doesnt mean the camera wont AF on its own anymore, it will, but if it doesnt, you can still press the shutter down and take a shot instead of having to reacquire focus and exposing yourself to miss that moment you want to capture.
If you have experienced the scenario mentioned above (or anything similar), its time to go to Release. If you havent yet, you will when you feel the frustration of a missed shot due to shutter lock.
Stop having missed shots because the camera didnt fire, have it fire ALWAYS when YOU want it to!