I understand, as a player, that an exhaustible resource, like source points, leads to a situation in many games where people hoard their "ultimate abilities" for a day that never comes (and thus all Larian's efforts to design these cool abilities are largely wasted as they are never used....). That's not very fun.
Likewise, I understand that Larian wants to keep source abilities feeling "special' and, to be honest, it's probably too late in the development cycle, balance and story wise, to just ditch the concept.
Why can't both points be addressed?
What if source abilities were regular abilities - unless source points were "spent" to empower them. For example...
Chain lightning by default would hit 3 targets. Using a source point would increase it to 7 and up the damage.
Epidemic of fire (or whatever) would hit 3 targets by default. Using a source point would increase the targets to 5 and inflict necrofire instead of regular fire.
Etc.