I don't see anything wrong with asking "why aren't you using scatter". I'm not trying to be accusatory, I literally want to know why they aren't using it. Knowing their reasoning makes it easier to explain why they should be doing it a different way. Like asking a MNK why he isn't using Rockbreaker on big trash packs, and when he answers "the potency numbers are bigger on my other skills" you can explain how in spite of the AoE skill having lower potency it still equates to more damage overall because you're hitting so many targets at once.

Or I guess you can cut out the middleman by asking and explaining all at once: "Why aren't you using Scatter? Even though it has low potency, you still do more overall damage than [X Spell] because you're hitting so many targets at once". Done. Tell them what to do and why, don't make it a passing suggestion without giving them a reason as to why they should listen. If they want to get indignant for someone asking a perfectly innocuous question, that's their problem.