Sometimes. If they have a MR3 item in the unit then no, However, I like it, as it allows my Slann to have more magic items. Note that the saves are only against spells.
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.
With the limit of one magic item per type and most of the things I want in arcane and disallowed due to cupped hands, I find it hard to take all my allowed points of magic on the slann. This means that if I want magic resistance I can normally use one of the talismans. I often got for the MR2 one as that gives me enough points for cupped hands and the Ring of Rhuin as well (depending on lord point army limits). The MR2 item gives the slann a 2+ ward the same as the discipline for 20 less points although the TG only get a 5+ ward. Also since I only tend to do this while running solo then the TG don't come into it and I get the same ward for 20 points less, which tends to leave room for ethereal.