I'd agree it's a useful effect because of the historical flavor and value, but the implementation is tricky. Tying it to tourism might be more manageable than to happiness, but if there are now tourism benefits throughout the game, then it's not clear we need it.
There's also the matter that it isn't really under your control in a clear way, when societies have often approached migrants with suspicion and restriction and constrained the ability of people of some location to come at all, "happiness" advantages or no and that isn't really included in the model either. Ideologically I think that's not a sensible idea for countries to be doing or to have done, but in game play terms, being able to focus on expansion instead of growth or on golden ages instead of taking in immigration matters.
You can get to familiar or popular pretty early on to provide some benefit. The benefits would be much less than later game, but just cutting the unrest and population loss or adding a few science from trade routes early on is not nothing. The immigration mechanic described above wouldn't have added much until around the same time in game terms in my estimation (renaissance or medieval say mostly).
There's also the matter that it isn't really under your control in a clear way, when societies have often approached migrants with suspicion and restriction and constrained the ability of people of some location to come at all, "happiness" advantages or no and that isn't really included in the model either. Ideologically I think that's not a sensible idea for countries to be doing or to have done, but in game play terms, being able to focus on expansion instead of growth or on golden ages instead of taking in immigration matters.
You can get to familiar or popular pretty early on to provide some benefit. The benefits would be much less than later game, but just cutting the unrest and population loss or adding a few science from trade routes early on is not nothing. The immigration mechanic described above wouldn't have added much until around the same time in game terms in my estimation (renaissance or medieval say mostly).