Would you rule the doubles bonus from Weapon Focus to be base or bonus damage to an attack?
My 4th level Thief with 10 STR has taken Weapon Focus (Daggers), and is the proud wielder of a Dagger +2. I have surprised a target, and roll to Backstab him. I roll a 3 for damage, and a 20 attack throw! Re-rolling my damage, it comes up 3 again. I grin evilly at my DM.
Do I do (3x2) + 2 + 3 = 11 points, or ((3+3)x2) + 2 = 14 points?
Honestly, Weapon Focus is rather underpowered as it is. I’d give them the extra 3 damage.
From an alternative perspective: “…the character may roll twice for damage and choose the higher result…” the implication is that the result of Weapon Focus is your damage, and that Weapon Focus amplifies it. takes off overly analytical rules-lawyering hat
However, RPG rules are rarely meant to be read that intensely.
Several backers have pointed out that Weapon Focus is under-powered. In v18 of the rules, Weapon Focus has been changed to do double damage.
If wielded in a backstab this would increase damage by one step (i.e. double to triple, triple to quadruple, etc.)