"If you can't say anything Nice, don't say anything at all."

[Airhead]

I don't really believe in that at all - there's nothing wrong with being nice, but I think it's equally bad to just shut up about everything that displeases you. Although it's unnecessary to be a dick for no reason, I don't see why you can't say things that aren't nice in a tactful way.

It's advice for children, and children like/need absolutes.

It's practical teaching because children are self-absorbed and don't really understand consequence in the same way...they also have a habit of stating the obvious. Your kid stating "you're really fat" or "you're really ugly" upon meeting someone, may be astute observation...but it's also very embarassing for the parents and the person who just got verbally abused.

Right, so there's this thing on one hand, telling someone "you're fat", just because. Now in this case it sure is better to just shut up. Fat people don't need to have their obesity pointed out on every occasion to know they're fat. Such remarks don't serve any purpose other than hurt the person they're directed at.

And then there's real constructive criticism, which is genuinely useful for improvement. If you made a drawing and everyone's just going 'ooh' and 'aah' over it, that doesn't give you anything you can use to get better. A negative but civil remark, like 'the proportions seem off' or 'you didn't get the perspective right' directs you to a problem you might have missed. So OP's saying is not something to follow every single time.

That's what I said too. Adults have the necessary ability to justify using their own discretion. The absolutism is for the kiddies.

Right, I quoted you post for reference, not because I disagree with you. Bottom line - the saying, as is often the case with sayings, is just too short and simple to be true.