Are -resist items helpful in raids?

-resist items are not really as helpful as it would initally seem. -resist items directly reduce the chance for your targets to partially resist non-binary spells and miss binary spells, by basically subtracting their resistance rating. However, warlocks already have built in debuff spells that lower the target's resistances by 75 (CoS and CoE), which is more than the resistance level of most raid mobs in the game.

The game also addes 8 resistance rating points for each level the mob is higher than you, up to 24 for level 63 mobs. These level based resistances cannot be negated by any means, including -resist items. Read "How do resists work?" above for more information on this subject.

So why are they in the game, you may ask. Well, obviously there are raid mobs with resistances higher than 75. In those cases -resist items would be effective. Secondly, people speculate that -resist items were meant more as a PvP stat. This would explain why nearly all of the -resist items are obtained either as battleground rewards (Rune of Perfect, Don Rodrigo's Band), or part of the AQ40 set pieces. Also, not all classes have an innate resistance debuff spell like Warlocks, so these items may be more suited for other classes such as mages, priests, and shamans.

In summary, -resist items are useful in reducing your chance to miss and be resisted, just not as much as you probably think it is.