Don't know either, but I definitely had the experience it helped. And super defense pots aren't exactly expensive.

 

So why not.

 

It is a certain percentage. May it be 10%, 20% , 30% or more. Whatever it is, it helps.

 

That's why I said you could compare it to wearing something with slightly better magic defense stats.

I´m quite fond of Spectre tasks as well. I have two ways of killing them:

 

1. Prayer (without potions, I consider using prayer potions on easy tasks a waste):

Setup: Proselyte armour, slayer helmet, Falador Shield 3 (it recharches your full prayer once a day, ánd gives the highest shield-spot prayer bonus), infinity boots, whip, fury amulet, spiky black vambraces. This should suffice not for a whole task, but for a few cheap kills easily.

 

2. Melee without prayer.

Setup: Slayer helmet, Armadyl chest and plateskirt, Iinfinity boots, black spiky vambraces, fury amulet, Elemental shield (if not using a gs or ss), whip. With that I could use about 20 cods (the noobiest food ever) for an inventory of decent herbs, all without potions.