Weakness potion effect on a mob does not decrease his attack power.

Steps to reproduce:
1. Execute a command

/summon Skeleton ~ ~ ~ {ActiveEffects:[{Id:18,Amplifier:15,Duration:2000000}]}

2. Change gamemode to survival
3. Enjoy getting damage from the custom-spawned skeleton
4. Execute a command

/effect @p 18 300 15

5. Take a Diamond Sword
6. Try to hurt any mob with the sword to check the power of weakness XVI
7. Have fun!