There's some missing of the point here.
If I see someone actually doing something dangerous, of course I would speak up. I hope we all would. Examples include Fuel-off's "seeing the preceding aircraft take off and had visible flame coming out the exhaust".
I don't speak up for something that may or may not result in a dangerous activity. For example, Shagpile's "there's a chance a little guy behind me is potentially unaware and going to takeoff or land into it". (Emphasis mine.)
Yes, it only takes two two seconds. So does every little comment, about every topic under the sun. Shall we each transmit our pet hobby horses at every CTAF?
If you see something actually wrong, say something. If you see something that might perhaps become wrong if the other guy is ignorant, forgetful or just careless, think twice. Maybe he is just as competent as you... Maybe that lighty is going to fly down a four degree profile and land long, because he's been sharing a circuit with turboprops for years now...