See Wiki on Spirit stuff
http://www.exiledkingdoms.com/wiki/inde ... esistances
Currently, spirit defense to protect against stun and paralysis. With equipment offering stun immunity and skills offering stun protection, spirit resistance is not as vital to survival as it once was, but it's still a crucial aspect of a build. Nothing like watching your character die a slow death after being stunned to remind a player how important stun protection is to a good build.
Weapons that do spirit damage (and cleric skills) are useful because spirit damage is a pretty versatile trait (that no enemies gets healed by).
Future updates might see more mental attacks added to the game (fear, confusion etc) but that's a long way off, if ever.