Grenades saved me a few times alerady doing T5´s and T6 quests with those nasty trap rooms that you drop into and can´t get out quickly anymore.
And yes demolition perk does give you quite a boost in damage. There is also the AtomJunkie candy. It gives you another 50% bonus damage for explosives. A grenade does 341 damage with an area effect of 5 blocks radius. With demolition expert maxed out that´s 512 damage. With the Atomjunkies that´s 768 damage.
Compare that to any gun wich can´t damage a whole group of zombies within that 5 block radius.
It´s a no brainer for me personally tbh as i am going heavily into perception anyways in my SP using the spear. Great crowd control aswell with the knock back, you can set up "traps" by dropping them at doors without pulling the pin and throwing another one there when they pass that door. ALso 100% stun chance with only 2 points in the skill.