Quote Originally Posted by Amphetryon View Post
Darrin speaks truth. CON damage is a damage boost and a debuff for your save-granting maneuvers and your allies' spells all rolled into one. It even makes Concentration incrementally harder, and you never know when that extra 1 point in your favor will swing a fight.
I would mitigate this by pointing out that there's a lot of things, as you go up in levels, that are immune to Con damage. Therefore, while very nasty, it can also, depending on your campaign, be situational.