It would put both the Indian and Pakistani nuclear elites in the dock, as it were, and would also be a way of these two smaller countries intervening in an issue that New Delhi and Islamabad would like to believe is solely their preserve. Such a stretching of the Bangkok Treaty would stimulate the general effort by the vast majority of the world's comity of nations to denuclearise the region and also politically undermine the 'nuclear credibility' of India and Pakistan. The political value of such an extension can be perceived by the existing P-5 which should have no objection to it. The other members of the Bangkok Treaty knowing of this general widespread support could with quiet confidence move towards endorsing the idea. There are no legal problems in carrying out such an extension of the Bangkok Treaty once all the possible political obstacles to it are dealt with.
Expansion creates no such material problems for existing members.
And from the Bangladeshi and Sri Lankan points of view, this extension does set a small precedent for exploring closer cooperation with Southeast Asian countries in other fields, something that India and Pakistan, given the 'miracle' performances of this region and East Asia, are themselves seeking to explore. In short, looking from all angles at the 'national interests' of Bangladesh and Sri Lanka, there are many good reasons and no bad ones for wanting such an extension of the Bangkok Treaty. Its offer was rejected by China and some other NWSs for this very reason. Given the landlocked character and lack of geographical contiguity with the Southeast Asian NWFZ, Nepal can consider another alternative - the Mongolian precedent of declaring itself a nuclear weapons free nation zone. Again, this would be a powerful rebuke against all its three nuclear neighbours, India, Pakistan and China.