It's just another annoying item to have to bring with you for slayer. Seems like it would just be a nuisance, since people would just forget all the time.
Well, then, you like how slayer is easy as it is?
Maybe, you should do more tasks, to see, what I mean. Bringing additional items for tasks is what Slayer skill should be all about. Not that I'm angry about your opinion, but, let's say, you get dragon task. Any dragon. Surely you will bring your antifire security, to take less damage for dragonfire, so, bringing anti-dragon shield, dragonfire shield or/and antifire potions is logical, yes? Then, you can get a task of any NPC who are in Smoke Dungeon, which, requires either Slayer Helmet or Face Mask, just to survive. Now, tell me, Captain Obvious. Should we remove those requirements as well, seemingly as it pointless for you to bring ANYTHING for task to be done?