I'm late to this thread, but I have always wondered about this as well, because sometimes it does seem very informal. This has been a very interesting thread.
As another instance of what I would consider improper address, I would point out Mr. Styles in BOT.
At one point he mutters under his breath(regarding the coded Romulan transmission), "...give it to Spock...", and then Captain Kirk made him repeat it.
Now, granted, Kirk dressed him down publicly, in front of the whole bridge crew, but not for the way he addressed the XO, but for perceived bigotry, assuming a Vulcan should be able to understand a Romulan code, by virtue of race.
But his dressing down did not appear to be for addressing the XO only by his last name without a Mister or Lt. Commander, or anything.
At the very least, he should have been put on report for this, if not relieved of duty, pending a disciplinary hearing.
Even though I love this ep, I have always questioned why Styles was allowed to remain on duty and not in the brig after the way he was insubordinate.
In fact, now that I'm typing this, I remember Styles also addressed Mr. Spock simply as "Vulcan" right at the end, when he was manning phaser control, "This time we'll handle things without your help, Vulcan".
He should have been court-martialed right then and there.
Walk the plank, Styles!