I do appreciate the idea of a 'Celtic church' is Fenian bollocks, that there was no doctrinal and little procedural divergence from Rome. However, the Irish church was long physically separated from the developing Roman tradition - there being no recorded papal contact with Ireland between Palladius' mission and the twelfth century. That's over 700 years more or less of sequestration. I know that by itself doesn't make a distinct religion, Ethiopia remained Coptic under far worse circumstances, but there were strong cultural overtones to the struggle between 'Irish' and Roman Easter long beyond the hippy vision of the Celtic church that make a separate sect desirable for gameplay at least through the earlier part of the scenario. I reckon it would be cool, say, as Northumbria to have to face Oswiu's dilemma. Perhaps stronger ties with Iona and the Picts, but I don't see Benedict Biscop coming to Tyneside had Northumbria adopted the throwback, thus no Bede, for example.
Ideally changes to/from CC would be solely by event, it would be crap if Spoleto or Vandalia could go 'Celtic'. Since CC would be broadly worse than RC too something needs be done to stop the Irish switching over prematurely as a matter of course.
But like Arthur, Mynyddog etc it's mainly crowd-pleasing.