I love the care put into this article and the responses!
Maybe this question is ignorant, but should the actual names of the cases matter? Should Sentence Case primarily be used for sentences, and Title Case be used primarily for titles?
While I sometimes enjoy the quieter (I prefer that adjective to “friendlier”) feel of Sentence Case titles/headers, I give a little less value and trust to something written in Sentence Case if it’s not written as a full sentence. And by full sentence I mean, at the very lest it should end with some form of punctuation. To me, if it doesn’t end with punctuation, then it’s not a sentence, and to format it as a sentence, on some level is a deliberate removal of structure/formality/completeness. Which is not unlike what you refer to as deliberately attempting to make a brand more casual or friendly.
I’m not against that approach—in the right hands. However, as a separate topic, I can’t help but feel our culture’s ability to communicate properly is deteriorating from too much casualness and lack of nuance and structure. Rough, incomplete blurbs of text permeate almost every email, text message and presidential candidate tweet we see.
Personally I like more structure in communication—it’s harder to maintain—requires a heck of a lot of care and attention. That I can trust. That, I can be more than friends with.