The only argument I can see against spin-offs is that the distribution of wealth in bitcoin is somehow "unfair." I don't think this view will garner much sympathy, however.
So you are saying that Bitcoin, a cryptocurrency whose users likely number in the hundreds of thousands at most, is the be-all and end-all determinant of how any future cyptocurrencies should be distributed? What would be the incentive for any new users who did not happen to hold Bitcoin?