I agree that secession was about the economy as well as slavery and state's rights. I would say even more than a significant part of the economy, and rather the primary system for economic advancement in the South. Keep in mind there were black slave owners as well as white ones (although relationships between the two instances were probably not always similar).
Of course, saying it was about the economy as well does not really give a separate reason from slavery since the two were intertwined. I also think that the state's rights argument was not just one of convenience, because to southern states slavery was an issue for states to determine, thereby making slavery just a single issue that the larger issue of state's rights would solve from region to region.
Slavery, of course, was a morally bankrupt social system and deserved to fail. But the South claiming state's rights as the reason is not totally incorrect.
