"If you can't say anything nice, don't say anything at all". Really?

[GruntledOne]

AIBU to wonder why people say this? Apart from anything else, I suspect MN would disappear overnight if this was the rule. So would history as a subject, and the entire criminal justice system. No-one actually lives by that rule, do they, apart maybe from people in religious orders?

My OH can get quite negative at times and it drives me a bit mad so I will tell him that the next thing out of his mouth has to be nice or positive to alert him to the negativity. It does have its place, I think.