Wall bouncing is incredibly useful in two main situations:
You have the health disadvantage- it will allow you get a body shot or evade till you recover health.
When you are outnumbered- it will allow you to get multiply body shots whilst evading or allow you to run away.
It also has many other uses such as getting a final shot on someone round a corner with low health quickly but without exposing your self to a body shot.
So anyone that says that wall bouncing is not a fundamental skill to being a great player doesn't know what they are on about.
As far as learning how to do it, it is simply a matter of knowing what to do (press a when you are close enough to a wall to slide at it, whilst sliding turn the camera so that you are facing another wall, use the left stick to pull out of the slide and tap a again, repeat) and practice, a lot of practice. Being able to slide continuously will come eventually, although being able to aim consistently whilst bouncing will take much longer.