That last example sounds like someone incompetent at email and choosing teammates, chose one who's incompetent at deciding what's critical. Oops-a-daisy. For competent adults, having the recipient unilaterally decide what's critical (critical to whom? being a key question) is insulting.
Yet still less insulting than those who conflate importance with urgency. Guess it comes down to the "competent adults" part; I've often found them to be few and far between at large companies.
Well, because it would be reasonable to assume that 40% of a typical business user received emails are something that should be ignored.
e.g. 10% of all emails are critical, important or urgent, 30% of emails aren't critical but need a response, additional 20% don't need a response but must be understood, and 40% are just "organizational noise".