Source points and magic armor/physical armor already discourage using your strongest abilities first, because source points are a limited resource, and magic/physical armor prevents the nastier effects of skills from taking place.
If there is one thing I totally despise about this game, it is source points required to cast a spell. Basically it makes spells you buy and learn the same as scrolls. A once use spell unless you find/buy another scroll.
We should need source points to permanently learn a spell, but not to cast it each time. UNLESS we automatically get source points back on enemies we killed.
My main concern is that later in the game we'll need to rely more on the powerful spells instead of the low level ones. Our complaints about "never enough AP" will be complaints about "never enough action points (AP) or source points (SP)". Instead of one complaint, we now get two

.
I guess the reason why I didn't complain about this before was I didn't get any spells that required SP to cast until right before the final battle. Used them once and ACT 1 ended.