Edit/Note: The weakness uptime builds sre actually quite useful, given that if weakness is provided by multiple group members using the Skale Venom where they could have used BiS consumables, which will be a significant dps loss for the group. A necromancer with any build from Brandon's thread can maintain weakness on both mobs and boss alone so your group can slot %10 damage increase or power increase consumables while providing comparable damage to Glass Cannon.
But again, Fractals are really the only place where you fight mobs that require Weakness, Keeping loads of potions clutters up your inventory and a lot of people don't want to do it, Warriors should be using Axe / Mace and Axe / Warhorn for more Weakness, so Weakness isn't really a problem. Thieves can apply it too, and those are also a staple in Fractal groups. I guess the main thing is keeping up with a lot of consumables, and frankly, it's just more convenient / doesn't matter enough not to use Skale Venom.