I think the key metric is that useful grammatical suggestions are ones that make the communication more clear. At the end of the day, clarity should be the focus. It bothers me when people make silly grammatical errors, but many of them end up in the final text because our minds auto-correct for them and as such they rarely pose much of a problem in terms of actual clarity.
I agree with you. The reason such errors worry me, even if clarity isn't an issue, is that the next generation reads these incorrect texts and assumes that everything they read was written correctly. As a consequence these kids misunderstand the grammar of their language. Lack of formal grammar training doesn't help. Because language is an important tool for thinking, degraded language skills lead to degraded thinking.
The example that always gets me foaming at the mouth is the persistent confusion of "are" and "our" in online forums, just because they sound similar in an American accent.
