I think this relates to council factions. Appointing a council of loyalists may be one way to test the notion. It seems reasonable that under most conditions councilors would pursue increased powers. The vassal limit boost it offers seems very substantial, though I haven't compared the base values with those from pre-Conclave.
It also balances abuse of council support requests; It adds some complexity to realm politics and allows vassals to more realistically obligate their sovereigns filling in some of the gaps in the dynamic and transactions falling between dominance and overthrow. Conclave rewards the ability to identify relatively opaque divisions of policy interests, securing in advance favors from characters that will come to positions of power. It also has the benefit of creating political objectives for fundraising.
The problem is just that one must surrender power to this, as pretty much all your strong vassals, on the council or not, will join it pretty much at game start, and keep joining it until it's lowered a significant degree. Regardless of player actions, this also heavily impacts the AI and it seems ridiculous that no power in the game can retain or achieve a weak council without the entire realm rising up, somewhat unrealistically. I understand the gameplay reasons, but still.