[ineedasername]

They are very effective in certain tactical situations. Area denial as an example. Or to slow an enemy. Or induce fear and morale loss. Or inflict countless non-lethal casualties that sap resources from the enemy: I have relatives who 100 years ago sat for weeks in hospitals trying to recover from mustard gas (they never did, not fully). Or to force the enemy to spend more resources negating them than you do to create then: The simplest are cheapest are easy to produce while effective gas masks for ever soldier are a little more expensive and not fully effective under active conditions of war. Use against civilian populations as a weapon of terror, as in Syria, is yet another one. Asymmetric warfare in general is a broad area of usefulness.

Certainly they were effective enough that Japan used them in thousands of attacks against Chinese forces. Effective enough that the US and Russia and probably others actively researched and stockpiled them for decades into the cold war. They were considered a significant threat by the US during the Gulf War: Iraq had a history of using them against Iran so the US took many precautionary steps to protect or inoculate their soldiers against them. Too many precautions as it turns out because those protection methods are themselves credited as a major factor in Gulf War Syndrome which impacts hundreds of thousands of US soldiers. The US certain believed in their efficacy 70 years after the trench warfare of WW One.

I don't know where you get the idea that they are not effective, or only effective under conditions if trench warfare. Actual use shows this is not the case.