I feel like they are only used by people who either force themselves to use them because they know they will have plenty of source points to spare later, or people who somehow get into a situation where they can't win the fight without pulling out a trump card in the form of an overpowered spell to turn the situation around.
You can compare this mechanic to having a 1 time use unique item at level 1 that instantly kills an enemy when used. You would carry it until the end of the game because you didn't want to waste it on anything but the final boss, which you still wouldn't use on because you'd want to experience the final battle like it was intended. Of course this is different because you can recharge the spells and they aren't instant wins, but the feeling of waste is kept because of the limited "charges" mechanic.
TLDR. You never need to use Source Spells, and it feels like a complete waste if you ever did.
A lot of this is just a mentality problem, not an issue with the actual mechanic. Saving special, limited use items/resources for the whole game is just something you have to get over. It's not a waste if it helps you beat a difficult fight that might've taken tons of preparation or specific or cheesy tactics to otherwise beat. I mean, invariably every fight will probably be beatable somehow without source points, but if it takes you 5 tries to beat a fight without a source point, and 2 tries to beat with using source points, then that source point just saved you a whole lot of time.
Of course, they shouldn't be so powerful that they're basically "I-Win" skills, which to a certain degree they are right now. I think many of them could be toned down, while perhaps giving you a few more source points (not requiring vampirism), so using them on non-bosses feels less "wasteful," nor as OP.
Another thing I think would help a lot is if fight win conditions were less binary. You either win your lose, and it doesn't really matter how well you did, barring costing some resources/gold if you use several items. But if there were more fights kind of like the Paladin one in fort joy, or Gareth, where you have to protect them, where there's more nuance to winning the fight and losing. Saving the Paladin/Gareth obviously are a better results than letting them die, so you'll want to play better and perhaps use things like source points to win the fight if otherwise they'll probably die.
But I think some fights could be even less binary. Like, for example, you could defend a farm from voidwoken who are destroying the crops. The more crops you save, the better the victory, and it should be very, very difficult to save 100% of the crops. And this should have RP effects, too, depending on your success, where food prices in the town would go up, and maybe some characters would starve or resort to violence for food.
Or there could be some kind of endless arena fight with repeatedly spawning monsters that only ends when you hit a lever to close the gates and kill the remaining monsters in the arena, and the longer you last, the more reputation and rewards you get, and plus it would be a nice way to compete with your co-op partners to see who could survive the longest individually, for example.
These sort of things would encourage the use of source points even in fights you could otherwise win without them, because you could win them even better with the use of source points.
