You hit it on a nail there!
Yeah, 19 tiles of mostly desert and mountains with zero opportunity to expand anywhere except in fluke games with Mongols and Chinese collapsing each-other...some Civ.
I've never pushed for Scotland, but they would at least have many coast-tiles, resources, and opportunity to take Ireland.
I think main reason they are not included is because they constitute a much to big "what if" factor for Rhye & Leoreth's game-design philosophy.
The whole 'Tibet independence movement' has been brutally, efficiently and very rapidly supressed at any sign of its appearance outside of Starbucks.
The seperatist movements of the Russian/Turkish/Persian-ruled Caucasus on the other hand...
Anyway. I oppose Polynesia because it would add negligably to overall gameplay and having them all represented by an individual leader is ridicolous.
You know what might be worse than having the pacific represented by one leader? Hundred/Thousands of HRE entities being represented by one leader or dozens/hundreds of Indian states represented by three Indian nations or a dozen feuding Italian city states represented by a leader...