Essentially Backstabber is giving you consistent damage at the maximum power, instead of the larger variation from the regular sword.
My example showed that average damage was approximately the same with Backstab included (but with the noted drawbacks). Strength weapons may have more or less variation, but I believe that is more of an issue of personal preference or situational as to which is better.
I believe that points into Scoundrel increase critical damage.
This is true, but Strength weapon users are pumping points into 2H, which gives 3% damage and 3% critical, and they have 100% crits from Rage. Scoundrel only contributes to damage with the 3% crit damage increase. This is another example of why the Daggers are underperforming.
