Well first of all the union is not joining the countries together. India is a strong democracy, China has no political rights and the rest of the countries are in between. The union (like the EU) is mostly to improve economic and cultural cooperation. It would be more beneficial to the smaller countries than bigger countries like India and China in the long term because they will get easier access to capital and bigger export markets. Just think about it, would'nt Pakistani businessmen make more money selling their leather jackets to neighbouring Indian or chinese market than all the way in EU or US?
And remeber whenever you refer to India, you also refer to 160-170 million muslims as well, and one of the few countries were muslims can follow and practice their religion with freedom.
Because of just having one of the largest muslim populations in the world, should'nt Pakistan acknowledge this and even support Indian to have member ship of OIC or other Islamic organisations or atleast have observer status.
By the way, we already have a economic union of sorts, its called the SAARC and hopefully its just the beginning. Not because it will benefit only India, but because it will help all South Asians.
P.S. Europe is not united in a single ideology either, they have divisions of orthodox, catholics and protestants. Remeber the IRA and mayhem there? There are strong diffrences between socialist, capitalist and ex-communist countries but they still work to gether for mutual benefit.