Holycrap Haha No. I'm sure you're right, and that many people would hesitate. I can say for certain that I would NOT. Oh, if this was a matter of personally going in to save the day, I'd be nowhere near the area. I may or may not get up the courage to at least try, at possible cost of life and limb; but no way it would be an easy decision. This, though? No way, jump in with both feet.
What's the worst that happens? Halloween averted, Xander does not know how to make bombs later, Mayor blows up everything. Incidentally, that only happens if you specifically withhold pertinent information, because forewarned is forearmed. They might let Halloween happen anyway, but have someone stay lucid to keep an eye on the town as best as possible to avoid fatal accidents. They might stop the Halloween spell, but look up bomb making in prep for the Mayor way in advance.
Of course, there are some things that would turn out better if they don't know. For character growth reasons, or because not knowing is a big part of the process, like with Dawn showing up. But that's hardly any reason to avoid helping where you can.