You are right. Thanks for your thoughts. I did say "formal writing." By that I mean the thing you send or submit for consideration by others. nowaypablo's essay, for example. Of course writing can be changed and often is, but when you write your letter to the grad admissions department (am_Unition) they are not going to give you a chance to fix it.
Your ability to conform to the minutiae of written expression is an elitist way for institutions to decide which sheep-goat combo they want to enlist. I want my students to, at least, be aware of those little things even if they don't subscribe.
I'm not even being hypocritical. I prefer to read without being confused or distracted by errors.