Also, Magical Resistance only works versus spells that cause wounds. Characteristic tests (like 'Purple Sun', 'Dwellers' or 'Pit') don't trigger it, so they'll still nuke you regardless.
The reason I would always take it is because Magical Resistance is hard to come by with wargear (one Banner, one useful example in the Talisman section), and so many armies rely on dakka magic to make up for their weakness at range. Actual shooting armies probably won't have much serious magic anyway, so it's really there to shield your Temple Guard from direct damage and magic missiles (which are still plentiful and deadly). For the Slann, it means he's almost impossible to snipe with magic (Ghennas Hounds, Death magic etc bounce off).
'Unfathomable Presence' may cost you taking Loremaster (never leave home without 'Rumination', that extra free dice is a god-send), but Plaque of Tepok is cheap and gets you pretty much the same ability.