I'll always get rid of a slow or paralyze effect, they typically have the most impact on the party's performance. I'll judge case by case if a poison spell is worth dispelling or if I should just heal through it. And I'll always dispel the misery debuff seen in Lost City of Amdapor, by standing next to them and using /comfort, because I think it's a cute mechanic. Sounds like you just see many unaware or lazy healers.