While multiple castings of Violation can be devastating, getting enough targets to cast multiple Violations is often tricky, especially against a savvy opponent. I've also run many a game with Sonnia where I didn't cast a single Flame Burst. But I still feel Sonnia is at her most effective when she can burn a stone for Confiscated Lore and then just do 2 or 3 soulstoned bursts (which is up to 4 stones in a single turn on offense alone) against a crew that isn't spread out enough (which, with 2" extra range and 3 is hard to do). If your opponent does spread out to minimize blast damage he's often at a positional disadvantage, so if you keep your crew tight enough together he'll have a hard time engaging.
So, in short: if you take a smaller cache of soulstones with Sonnia you're limiting her potential, because if you only have 4 - 6 stones, would you really be willing to burn 4 of them in a single turn?