With the old SP system, few enemies could actually be fought by parties because of ridiculous disparities in enemy type (they had to have a lot of HP and strong AOE.) Most enemies were ignored because of bad SP gains. This is a bad thing. I should not run through a zone saying, "Wow, look at all these dangerous-looking and awesome enemies that aren't worth killing because they suck for SP gains. Where are my crabs, man?!"

And what, pray tell, makes the current system impossible to balance? In any system, people will find out what works best and use that. An implementation of an SP chain would work in the system we have now too, so there's no need to limit us in terms of enemy choice.