As far as I know, after the first few levels, most people don't run around with a bar made of skills from only one class. So this got me thinking, why does only one class (your current) get SP and not the ones you used skills from.
Example, I'm a lancer and I've got radiance, cure, and blizzard from my conjurer class. If I use those skills, what if some SP go towards my conjurer class. If I then add second wind from pugilist, it could get some SP as well.
So what I'm suggesting I use skills from another class that class the skills come from gets a few bonus SP (10%?).