I found an interesting post about a Tamilian's plight with desis talking to him in Hindi and here is what I told him: (For the post look <here>
Well, I am from Madras and I had a similar view about Hindi until I landed here in the US. Even though I had taken hindi as my second language at school till the 10th standard, I really couldnt speak in hindi. I will be forever grateful to my roomies who took the pains of conversing with me in Hindi and teaching me several things in the language. Mind you, all of them had their own language (telugu, kannada and marathi) but Hindi was our common language. It should be our common language, because it is the national language, plus people are more comfortable talking in Hindi with desis than english - which I think, is very understandable. If I meet a tamilian I would be more comfortable talking to him in tamil than in english - unless of course I meet a Peter or a Mary! It is only in Madras that people dont talk in Hindi (it is now getting better and better every day), among the major cities in India (take Bangalore or Hyderabad, for instance).
Coming back to the point, the basic problem with us tamilians is that we tend to think of Hindi as an alien language and do not even want to make an attempt to learn it or use it (I had this exact feeling till I came here to the US). Our politicians are the only one to blame for this - but they dont have to go out of the state to interact with people from other parts of India, it is people like us who have to do that on a daily basis. And it is in our own personal benefit that we learn the language. From my personal experience, learning hindi really gets you more friends - in the US knowing hindi is very essential, it puts the other guy at ease when he talks with you. And after all, if you dont speak hindi who do you think is on the losing side - you will be thought of a typical madrasi (stereo-type) who doesnt want to talk in hindi.
A funny incident happened when I travelled to Rameswaram. We had to book a hotel and the SethJi over there, demanded a higher room rate than what we had been told earlier when we checked in. My Father (who doesnt speak hindi) tried to talk to him in English and in Tamil, but nothing worked. Then I went and talked to the SethJi in Hindi (All I told him was, when we came in we were told of a lower rate, and this was all in hindi), and immediately he agreed and my family was looking at me with awe!