Yeah, that's true for Cardinal but heretics can appear anywhere, sure you can denounce them but look, you're not going to travel half-way across the map to denounce a single heretic, aren't you? At least that's not how I play. Okay, you might have several cardinals, but what are the odd chance of having many of your priests selected as cardinals...well maybe, if you can take out enemy cardinals...wait, you need to kill them...with assassin! You still need assassin!

Anyway, those priests are walking slow, try to compare them with assassin, assassin has greater walking distance, and therefore can take out targets quicker and more importantly, unseen by enemy. Let's assume you use priest and failed then turned heretic, now you have more troubles to deal with.

Well, that's just my opinion anyway. And no, I'm not anti-priest, don't accuse me of being heretic