I don't know about necessary, but it was expected. All three movies were going to be made and those that read the books already know what happens. I think it will be interesting to see how the book was interpreted and see if the style from the first movie is still present in the films. Also, I think people generally care about Jennifer Lawerance's portrayal of Katniss and want to see the story finished out.

I definitely am in agreement with the above stated. :D
I personally always relish the prospect of being able to experience a book coming to life, as it may elucidate some of the content of the book that was formerly inpalpable to me, or just to see how actors, both illustrious and fairly new, portray their assigned characters. :D Xxx
Nevertheless, I have established a correlation in that, from all the movies and sequals I have encountered, the first movie usually tends to be better. :D
However, we shall see whether 'Catching Fire' corroborates itself not to apply to the above I have stated. :) Xxx

Well, The Hunger Games is based on a trilogy of books, so it's near a given (especially with how big The Hunger Games and its fanbase is) that the rest will be made.

Because the movie is based on the trilogy of books it seams wrong to not make the other 2-3 movies

I wouldn't say completely necessary - not the way Mockingjay is necessary, the first book and film didn't end on a cliffhanger. But it's always irritating when a story continues but the films don't. And it would have been stupid of them to throw out the opportunity to make more money from it.