Overall, a bile is pretty bad because you can already have that effect with an Elixir or Food (bile doesn’t stack on top, you don’t get +45% but only +30%), but you’re losing out on whatever damage you could have had from a kit that boosts raw stats.
On top of this, even without using Elixir or Food, if you have full Strength/Agility attributes, each point of damage from your weapon is doubled when wielding it, if you also have armor that gives dmg bonuses, your weapon’s raw damage is even more important!
Very rarely will +15% be superior to +9 damage, simply because this +9 will be +18 or even +20… For this example, +15% would barely be superior to +9 damage if the weapon’s base damage was already 134 
→ And let’s not forget you can already get the +15% after with elixir/food, and this +15% will also take in account the +9 damage giving you +1.35 damage from the +9…
Well you get it, bile of Strength isn’t good if you have proper equipment.
The only reason to use it would be if you run around naked with no attribute points, with no elixir/food buff…
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