I would agree with you to some extent here; at very least, it’s well-worth learning the standards of formal writing even though it must be understood that these standards are in many cases artificial. But many of the rules that get passed along here are simply erroneous - that is, they are not actually used in formal writing. The old example of rules about “split infinitives” (a misnomer, incidentally) is one of those; examples of infinitives being split in formal writing are abundant. Violation of the “rule” about which and that is a well-honored tradition as well. Forgive me if I’ve mentioned this before, but it’s one of my favorite examples of the silliness of the prescriptivist brigade. When E.B. White decided to update and edit The Elements of Style, written by his old professor, William Strunk, he added in the rule about restrictive and non-restrictive clauses, and undertook to edit out several examples of Strunk “violating” that rule in the very same text! Unless Strunk wasn’t writing “formal English”, it seems rather presumptuous that White would have decided to “correct” his old mentor’s grammar. But that’s how these rules work - they are grafted onto language quite arbitrarily, and it must be remembered that many of those who critique others for these “errors” are found to make them themselves.
Being able to write in formal English is necessary for anyone who wishes to pursue a professional career; however, a great number of these prescriptive rules simply don’t accurately describe eminently formal (and elegant) writing at all.
I can’t quite parse this sentence. Are you implying that “It is I” is correct? It’s certainly not. It comes from an 18th century misapprehension that the English copula, be, ought to operate exactly like the Latin copula ESSE, and that therefore the items on either side ought to be in the same case. Nonsense, of course, as English is not Latin, and that rule has never been a part of any register of English usage (either before or after it was proclaimed); certain people wishing to appear even more scrupulously “correct” than their colleagues might use it, but it’s a self-conscious and unnatural style that is contrary to the norm of formal English usage. You may certainly do it; you may also ask if there are any remaining “agenda” at your next faculty meeting (since that is, after all, a Latin plural!) You may pluralize “trauma” as “traumata” if you wish (that, at least, was indeed the prevailing English usage at one point, unlike the other two.) However, be warned that other people will laugh at you - and quite rightly so; there is a long and beautiful tradition of laughing at people who try to show off with language.
“It is I”, though, in contrast with many of these other rules, is not just fussy or overly restrictive; it is, quite simply, an incorrect usage, and one that does not exist except in the most self-conscious and artificial speech. Another language’s grammatical rule was falsely held to apply to English, even though that rule contradicted the very natural English habit - one that, of course, existed long before (and will obviously outlast) the Latinate affectation - of treating the second item as an object of the verb. It is simply false to imply that your affected Latinity is “correct”, as it violates the long-held rule of the language you’re speaking.