Doing research on the weakness effects because they weren't really that important prior to 1.9 update and now they're going to be useful in PvP and PvE fights. This also maybe a WAI but it seems a bit odd.

Standard Weakness Potion:
Reduces Malee attacks by 4 damage.
Effects command
Sets duration of Weakness with a multiplier (assuming it's the multiplier of the base value, which in 1.9 is 4).

How to reproduce

1. Use a wooden sword: "4 Attack Damage"
2. Drink a Weakness potion: "-4 Attack Damage"

   /give @p potion 1 0 {CustomPotionEffects:[{Id:18b,Duration:1200,Amplifier:0b,ShowParticles:0b}]}
   

3. Try to hurt a mob
   It will not work
4. Use a wooden sword with Sharpness: "5 Attack Damage"

   /give @p wooden_sword 1 0 {ench:[{id:16s,lvl:1s}]}
   

5. Drink a Weakness II potion: "-8 Attack Damage"

   /give @p potion 1 0 {CustomPotionEffects:[{Id:18b,Duration:1200,Amplifier:1b,ShowParticles:0b}]}
   

6. Try to hurt a mob
   It works even though it should not

Old reproduction steps:

Test scenario
Splashed weakness potion on me and a Pig.
Used a wooden sword. Couldn't deal any damage (as intended and expected).
Used diamond sword, dealt less damage (as intended and expected).

Changed to the effect command:
/effect @p weakness 67 60 (weakness for 67 seconds with a 60 multiplier)

Used Wooden Sword, Couldn't deal any damage (as intended and expected)
Used Diamond Sword, couldn't deal any damage (as intended and expected)
Used Sharpness II Diamond sword, dealt reduced damage (not as expected)
Used Sharpness III Diamond Sword, dealt reduced damage (not as expected).

*Changed command: *
/effect @p weakness 67 200 (weakness for 67 seconds at 200 multiplier).
Changed Mob
Switched to Chicken (4 health points, 2 hearts)

Used Wooden Sword, Couldn't deal any damage (as intended and expected)
Used Diamond Sword, couldn't deal any damage (as intended and expected)
Used Sharpness II Diamond sword, dealt reduced damage (not as expected)
Used Sharpness II Wooden Sword, dealt same reduced damage as Sharp II Diamond sword (e.g took 3 hits to kill the Chicken; not as expected).
Used Sharpness V Diamond Sword, dealt the same as other 2 sharpness swords (took 3 hits to kill Chicken).
Sharp II Wooden sword lists as an attack damage of 5.5.

I do apologize if this has been discussed before. Search didn't bring up much concerning this.