A lot of lore was changed between Daggerfall and Morrowind, introducing the Imperials as a distinct people was one of them. Before than, the "Imperials" were humans of Nordic and Breton ancestry living in Cyrodiil with a distinct culture from their native provinces, using their more cosmopolitan culture to establish the Empire, though the Cyrodilic Nords were the ones with the most power under the Septims I think. Then a lot of things changed and the Cyrodilic race was born, Tiber Septim became a God and the Tribunal appeared and lots of other things.
As the lore is now, the Imperials always existed, being the descendants of the nedic people the Ayleids enslaved, but the lore changed significantly after Daggerfall.
As for the thread question, I don't like them that much. They have always been sort of a weaker version of the other humans at just about everything. Supposedly good at trade and diplomacy, but I don't buy that. All races have good traders, since trade is common in every nation. The Morrowind description even portrays the Nords as rather enterprising in trade. Then there is diplomacy, which there is only one good example of being used in the Septim Empire, which was during the War of the Red Diamond. Since then, Imperial Diplomacy has really meant "under the Imperial Legion's boot", and their own province commonly divide into petty kingdoms when there isn't a strong ruler to force them all in line.
In gameplay, they're not really good at anything, in any of the games they are in. Weaker fighter than the Nords and Redguards among the humans, and worse at magic than the Bretons. Their stats doesn't make them any better suited for stealth than the Nords and Redguards either. In Skyrim, they have really become the Nord-1, given the utter useless passive racial ability(2-10 more gold in some containers compared to 50% Resist Frost) and the nearly identical activated racial ability(fear and calm, both being slightly different forms of crowd control).
I like the idea of an "Imperial/Cyrodilic" cultural identity made up of different races living at the center of Tamriel, caught between the other groups, but I don't see any benefit in making them a race of their own. And I don't find much reason to play as them.