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?
They can mine the new coin mining starts at block 0. Also buy it cheep if they believe it valuable when others dump it.
There may also be incentives like high transaction fees with low difficulty to incentives mining and reduce the flood of liquidity.