At first I suggest you to fix your English.
It will be good for poor countries but overally - bad. Let me explain: yeah, we all dream "at least claim we dream" beautiful world with full of kind people. That's not a deal. One currency would make life easier for poor and importer countries which can't produce their own products and for example USD rise can affect their economics a lot in a bad way.
But now think deeper - no rival = no ambition = no development. Why will I work for others? Poor countries automatically help economically developed countries. For example companies produce their products in china because workers are very cheap here so company owners can offer reasonable price and get high profit. It's like symbiot, really. Poor countries can't create anything important so they need strong countries and oppositely.
Only one currency would ruin everything.