A quick clarification: It only lengthens conditions that you apply, even if someone or something else causes the same condition.
Imho - in PvE, except for Silencing, most condition lengthening mods are overall less effective than direct damage mods like Vampiric or Sundering. Most often, if things are going right, the foe will die before the condition runs out, or the condition will be removed, or you'll have an easy opportunity to re-apply it. Meanwhile, you could be doing extra damage right now by using Sundering or Vampiric.
Lengthening Silencing is more effective because Daze is usually relatively short, and it has the benefit of reducing the target's ability to do damage.