[VedalkenBear]

Apparently, I'm the only champion of the heretics here.

For Serpent Cult Pythium, they are the source of your wide magic diversity. In particular, a Blood Reveler gives you access to one of your best national summons (the Satyr you get from Orgy), as well as starting the 'blood chain'. Finally, as others have said, you can take very bad scales and remove them from where you don't want them using them.

For _anyone_, they are very useful in removing enemy dominion from your territory. Yes, they aren't stealthy. They don't need to be. Have them with your advancing army and watch your opponent's dominion drop. On p.94 of the Dom3 rulebook, it states that the Preaching command (which is presumably what the heretic uses every turn, at their Heretic level) gives a chance of increasing dominion in the province equal to (30%*Holy)-(5%*Dominion).

This means several things. First, the heretic doesn't care about your dominion strength (like temple checks do). So if you plan to have them in your dominion (and your dominion is positive), make sure your dominion strength is high to lower their chances of lowering your dominion. Second, since most Heretics are the equivalent of Holy-3 Priests for their 'preaching', they have a 40% chance of knocking any dominion down a point, and it snowballs from there. If you have 3 in a province that you recently took from an opponent, and the dominion is -10, the following table gives you the chances that the dominion will be lowered (assume that the preaching checks are done sequentially):

0 points = 0.6^3 = 0.2196
1 points = 0.4*0.55*0.55 + 0.6*0.4*0.55 + 0.6*0.6*0.55 = 0.451

and so on. In the case that they are not done sequentially, then it is a simple binomial distribution. Finally, considering they are primarily found in the Late Era (the Serpent Cult), it's a valuable way to fight the dominion of Ermor and R'lyeh.

So yes, they definitely have their uses.