Ill give you some reasons to trench instead of building walls:
1.Walls can go down easily

The guy in the trench is safe cause there are lots of blocks and the angle is too small.

The guy behind the wall is ducked cause his wall can be brought down by removing one layer of blocks in it.To do so you need about one SMG magazine
2.The enemy doesnt sees the Trench instantly

You see that the wall is made out of coloured blocks that draw lot of attention to them(Why do People build coloured walls?).
Youll expect an enemy behind it and be prepared.
When you see a 4 block long hole in the bottom(that doesnt gets your attention like a duckin pink wall)you wont be instantly expecting an enemy.
Another thing is that lot of people use weaponmods and most of them are longer then one block.
So they know your position instantly.
3.Trenches are griefersafe

As you see your wall can be destroyed by some griefer easily.

But he cant grief a trench.
So I hope I could get some of you to digging trenches with my little post why trenches are superior to walls.
Happy trenching guys :D