For a Monk the wisdom score is not only an AC boost but also the score that is added in several of the monk specific feats (e.g. just like strength adds damage in melee and charisma adds to the turn undead ability).
As of right now I can't remember exactly which feats/skills/abilities the wisdom score applies to (it's been a while since I played a Monk) but it is the "recommended score to upgrade" (by the game) when reaching level 4,8,12,16,20.
Druids, Rangers, Clerics and Paladins use Divine spells and they are exclusively dependent on the Wisdom score. Clerics and Paladins also get certain feats and abilities like Turn Undead and Smite Evil. The effectiveness of these feats/abilities are based on the Charisma score. Since a Cleric can cast 9th level spells at higher levels the Wisdom score is by far the most important for a Cleric (you must have a natural wisdom score of 10 + spell level = 19 to be able to cast them and equipment enhancements doesn't apply in this case). A Paladin will never be able to go further than 4th level spells and even though a high wisdom score will mean more spells per day, the trade off is a lower charisma score and this is where the Paladin usually stands out so it's not really worth it.
Neither the Ranger's nor the Paladin's spell repertoire is a match for a Druid or a Cleric but their enhanced fighting prowess makes up for it ... though the multi-ability classes (like Paladins, Rangers and Warlocks) are a little harder to play than a strength boosted fighter or an int boosted mage or a wisdom boosted cleric.