I think the nuclear deal is very good for India. I also think what the left is suggesting is fair. They have an opinion and they can express it. A good discussion reviewing the merits and de-merits would be good.
It is said that the Left could be doing this out of pressure from China. But, "the Left" being Indians, I would trust them first and would like to listen to why they dont agree with the deal and what they think is bad about it. It is never a bad idea in a Democracy to have a discussion, to listen to opinions that are being expressed and to especially pay heed to the people who have rightly been elected as representatives of one or more sections of the society.