The chaff is mostly there to prevent your soft glass cannons (Blade Dancers, Impalers and even Abyssal Horsemen) from getting charged and taken out before they can get their alphastrikes in. I find both elves and twillight kin to be pure alphastrike armies now that archers got nerfed so hard (that they are just a waste of points… don’t take them). They definitly don’t win any grind battles, so they have to take out their enemies quickly.
The spears are just cheaper steady points which can take a charge and also dash out some damage… but most importantly take objectives. The key to the army is to kill off more than you lose, and that means using a combination of chaff and the alphastriker units well.
My recent lists has included 4 gargoyle troops together with a mix of 4 choices of blade dancers, impalers and abyssal horsemen (and usually at least 2 crones and a bit of different stuff otherwise), and I feel like the three damage dealer choices are quite good. Expensive yes, but with the rather cheap chaff you can often outdeploy your opponents and manage to punch a hole in the defenses at some weak point of the enemy army.