Just a philosophical point, but if people are interested in tweaking this power, which has a different cost based on the PCs ability scores (so Constitution-based warlocks pay less or nothing compared to Charisma-based warlocks), are people also interested in changing powers that have differing benefits based on ability scores?
For example, consider the monk's Purifying Meditation, which allows a save "...with a bonus equal to [the character's] Wisdom modifier." That is quite different for a centered breath monk (Dex/Wis) vs. a stone fist monk (Dex/Str). One gets what amounts to an almost-automatic save, while the other (most likely) just gets a save with no bonus. Other examples might include the sorcerer's at-will Lightning Strike, which is much better for a Dex-secondary sorcerer than a Str-secondary sorcerer.
I'm not sure which side I would fall on. The warlock in the campaign I DM didn't pick that utility (and is Charisma-based, anyway). I'm just wondering if the idea is that all powers should be balanced, regardless of which attribute(s) they refer to, or not. If people aren't interested in balancing all of these powers that have different riders based on ability scores, then why is this one different? Is it because it is a cost instead of a benefit? Just food for thought, I suppose.