It's a nerf as much as a buff. Added weight means slower movement speed. It also means better knockdown/weapon stun.
If someone is focusing on speed or a ninja build only, this added weight is indeed a nerf! So it's kind of neutral overall.
It would be cool if we could choose which attributes to increase on a weapon. +1 = more damage. +2 = More speed &more damage OR more speed & more weight. +3 = more damage again.