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.