I run a similar build to this and, while I cannot speak for the original creator, I don't run Backstab because there are far superior talent options. Backstab is situationally useful at best. While it is true that you can get a massive first strike, it often leaves your rogue open and exposed and, therefore, is more of a micromanagement hassle than anything else.
For those running a build like this one, I strongly recommend acquiring the unique Stiletto found on the blacksmith in Act 1. The proc "Static Strike" absolutely OBLITERATES things. I am currently in act 2 on my rogue, playing on hard, and the game has almost become too easy. I'm sure I will run into something that hits extremely hard and will be forced to reload, but right now I am flying through things. My largest single strike with a stiletto is 83. However, I levelled a "named" mage for 123 damage earlier today. A basic strike of 50+ and a static strike proc of 70. They happened simultaenously. My other stiletto has the draining modifier, restoring endurance on hit, and it makes my quite challenging to knockout. Especially since I can always use blinding strike when needed.
I recently came across a strong ring with the "second chance" modifier and placed it on my rogue. It is probably responsible for 80-90% of my group's damage. Not because the rest of my party is weak, but because the rogue is so damn strong.