Why Sacred Exorcist?
While better BAB is nice, the better boon is turn undead. There are several powerful divine feats (Law Devotion, Travel Devotion) that work very well for a gish.
Seems like a bit of a waste. Also, BAB is NOT important. Ever. Arcane Disciple: Anything With Divine Power, and bam.
While this is nice in theory, in practice this approach has several limitations.
1. MAD: You need the Wisdom to cast the spell as if you're a cleric. 14 Wisdom on an already slightly MAD character can be difficult. Sure you can overcome this with a +4 Item, but then you're spending a significant chunk of money.
2. Action Economy: Divine Power is 1/round per level only. Spending actions buffing means you have less actions left for winning.
3. Limited Use OR Opportunity cost: Prepared casters can only memorize one domain spell per spell level each day. Spontaneous casters must still use a spell known to learn which limits them further (there are plenty of great level 4 spells).
4. Feats are a very limited commodity. Gishes are already trying to fill multiple roles in addition to many good gish PrCs requiring a feat tax.
Point 2 and 3 can be overcome by prepared casters that can find a way to apply Persistent Spell for free.
While Arcane Disciple is an option, it's certainly not an auto-include by any stretch.
Also, OP, you never answered my question, why are the divine PCs not tanking? They are better suited for it.