Well there may well be intellectual advantages for multilingualism, different unilingualism typically just leads to conflict.
not necessarily, there is often a clear split between social classes of groups of people (i.e. consider the association of "cockney" with working class, or the discussion around "ebonics" for african-american-english)
And there are cases where specific communities evolve their own sub-languages (e.g. cants and argots).
There is surely less separation now then there once was, but it never was only geography, it's just that geography is the strongest separator.
And it might mean some people get the chance to improve their own countries too, instead of being forced to immigrate (by globalization pressures) and try their luck on another.
In the age where importance of geography is diminished, and with so many channels of communication for both text and voice, a trend towards a single language seems inevitable - there are just too many things that would benefit from it, from science to commerce, and to some extent even art (you trade diversity for audience).
The biggest factor in dialect development is time, which is why you see a large variety of dialects in England as compared to the USA. England is much smaller, but has had much longer for dialects to develop.
They are. There's immigration, but that's just a tiny fraction of what goes on. We don't move randomly around the world. Even the immigrants come to a place to settle down and start a family and be integrated eventually.
And different countries are also separated by different outlooks on the world, ideas, ways to each. Which is a good thing (and orthogonal to war etc which historically its more about interests -- war can be just as brutal between brothers and similarly thinking people, as civil wars have told us).