My bad

o
But yes, the AI absolutely gets coalitions. Its just that the AI knows to constantly spam its diplomats to improve relations with everyone around, something I don't think a lot of players will do.
Not only does AI get coalitions, you can use coalitions against AIs to expand. Steps:
1. Ally a relatively small nation and bring them in on an adjacent bigger one.
2. Let the ally occupy as many provinces in the war as possible.
3. In the peace deal, give 4, 5, however many provinces you can to the small nation.
4. Vassal the target in a follow-up war (15 AE), break your alliance, join the coalition against your former ally, and force him to return cores to your vassal. PUNISH him for his aggression!
5. Annex your vassal and blob like crazy for 15 AE.
It is? I certainly hadn't noticed such an effect.
Base tax is not directly relevant to AE in any discernible way. I find it ironic that many coalition proponents actively demonstrate a lack of basic understanding behind the mechanic.
From an AE standpoint, it is more efficient to cede high base tax provinces rather than low ones, because you take fewer provinces due to WS limitations.