One indeed wonders how there exists so many languages in India. To me also it is very surprising how so many languages could have developed in India. In India, as anywhere else in the world, civilisation must have grown around useful earthly features like rivers. In the ancient times, these pockets must have developed human societies. Each of these societies must have had the need for communication. Thus, they would have developed their own language. However, we have evidences that Indians traded to far distances. So, it is also natural that people within these societies must have interacted with each other. So, the only plausible hypothesis seems to me that each pocket of human development must have valued their personal identity as the foremost entity for existence. What confuses me is that why should this be seen as essential part of the formula for survival on this earth. This confusion can be understood on other aspects like food, clothes and shelter as the material available would have been different in different regions. Then, what was the need for retaining the languages.
However, the fact is that this diversity in languages is not restricted to India alone. We feel this diversity in India as India is a large country where so many diverse pockets have learnt to exist together. Many would agree that India is actually many countries put in one. What India achieved in 1947 was recently achieved in sorts by Europe by the formation of the European Union. As in India, when I travelled across the European Union, I had problems in communication on reaching France from Belgium. Similar situation arose when I travelled from Antwerp to Brussels. To a lesser extent, language caused concerns when I travelled to Cologne from Antwerp.
What I am trying to prove from the above argument is that the situation that is India with respect to its being multilingual is not unique to India. It is the same everywhere else in the World. So, as far as I can see, being multilingual is neither an advantage nor a disadvantage to India.