I don’t look at the dark Apostle as 72 points because you should really always take the disciples, so he’s basically 82 pts and a bare bones sorcerer is 88. Unless I physically can’t scrounge up the six points I will take a sorcerer every single time.
The main problems with the dark Apostle:
1: Prayers happen at the start of the battle round, so both he and his target have to be on the table, not in transports or deepstrike.
2. Unless you’re word bearers, you only get to chant one prayer. (Don’t get me started on how
lame it is that loyalists get master of sanctity for any sub faction, and ours is locked behind *word bearers*
3. If you’re taking dark disciples (which you kinda have to, unless you realllly trust a 3+) he takes up three transport capacity.
He does have a perk or two over the sorcerer:
1. Prayers can’t be denied by your opponent
2. With the dark disciples, and even a little less without, the prayers are more likely to go off
As others have said, he’s a support character so you have to consider what units you’re trying to buff. If you’re using him to protect a forgeworld dreadnought parking lot or some other stationary castle he might not be the worst choice. If you want him protected from snipers or if he needs to keep up with advancing units, you might want to look elsewhere.