The quote that vectormane speaks of is one of the most oft mis-quoted quotations cited about the statesman. The entire paragraph (coppied from the complete text at http://bc.barnard.columbia.edu/~lgordis/earlyAC/documents/observations.html) is:
And since Detachments of English from Britain sent to America, will have their Places at Home so soon supply'd and increase so largely here; why should the Palatine Boors be suffered to swarm into our Settlements, and by herding together establish their Language and Manners to the Exclusion of ours? Why should Pennsylvania, founded by the English, become a Colony of Aliens, who will shortly be so numerous as to Germanize us instead of our Anglifying them, and will never adopt our Language or Customs, any more than they can acquire our Complexion.
What Franklin was speaking on here was not about keeping aliens out, but about not keeping them separate in society. He had seen, in Britain and France, what happens when you force people into a demographic, and as such was against the idea of cultural separatism. Therefor, Franklin was specifying that one of the tenants of some great future nation should be the melding of different races and cultures instead of insisting on separatism. He was a great fan of the 'melting pot' ideology; whether you agree with said ideology is up to you.