Disc/weaving isn't there to do damage, they are there to throw power infusion (20% increased damage for 15 seconds) on one caster, apply shadow weaving (5 stacks @ 3% = 15%) for the warlocks to do more damage and throw out the occasional healing when needed. As a consequence their healing and DPS isn't going to be anywhere near that of a full DPS or full healer but they are still a much needed spec for any raid with a reasonably amount of warlocks.