In reality everyone was designing there own nuclear program, USSR, Nazi's, America. So why should one do all the heavy lifting and let all other civs benefit from it while they save up there production to build other stuff. Because your culture benefit isnt exactly that much of a deal with this wonder and by this time it will not impact heavy. So while u got 1 city working on the manhatten program, other civs can juz build up units or other wonders and such. And when you complete it, everyone gets the beneift wich is nuclear capacity, plus the stuff they ahd gained from not having to lift a finger for this wonder. So i guess it would be more beneficial of it was a large wonder to juz kick back and wait till someone else makes it for you. And if a smaller civ isnt able to make it fast enough, well then they get overrun, its survival of the fittest in Civ and always has been, and they are clearly not fit enough. So cry havoc, and let slip the dogs of war.