Quoting Curseman,
reply 3
I think it would be easier to balance if they made it so % boosts only stacked with your character's base attack rather than with items, because if they do that, then it's almost inevitable for either the skill +items to be overpowered, or the items and skill separately to be underpowered.
No it means the +damage items would be overpowered for the rook. It means as the Rook you might want to consider using +str items instead of more health/armor, or lower cooldowns, if you use this skill because you'd get a better benefit for it.
If it effects the base attack and not items, instead of the items and not base attack, then there is no such thinking to be done.
Thinking is good, IMO.
You still have to think about whether you want offense, defense, speed, utility, etc when you're out purchasing items and getting skill upgrades in either case. The difference is that if SoG stacks with items, then they can choose to balance it around items not being used with SoG, in which case SoG + items will be extremely powerful, or they can balance it with the assumption that players will take both, in which case they are likely to be weak separately.
If taking +damage items is the best choice for the Rook, then it doesn't encourage more thinking. Having one option better than the others means that there's no thought needed, just take that option. If there are several options, and it is difficult to decide which is best for any given situation, then it takes more strategy.