A general ergonomic rule-of-thumb is to adjust your monitor's vertical position so that the top edge is level with your eyes and you don't need to look upwards.
Do you go around believing every lazy-ass statement you've ever read?
My gut estimate is that I actively view the 20% of my portrait monitor above my horizontal line of sight about 2% of my total working time. What's up there, anyway? A menu bar, a window title bar, a bunch of FF controls, a bunch of FF tabs, the Slashdot header, some junk about DEALS NEW, "Reply to: Re: Have Both", then the Slashdot story header which repeats "Re: Have Both (Score: 3)", then there's you user information / date / perm-link. Everything else on this screen is below my horizontal line of sight, including the entirety of this input form where my gaze is normally focussed.
That lazy-ass statement almost certainly originates from an era where devoting 20% of a monitor to menu/window/media cruft left you with a painfully small working area.
If you bother to read articles where researchers are interviewed decades later about lazy-ass statements they tend to say: "well, yes, of course we knew that at the time, but at that time hardly anyone had even heard of ergonomics, so we chose to make the message as simple as possible, so as to get 80% of the benefit from 20% of the yammering". Last time I ran into this it concerned one of the BMI formulas (there are several body mass formulas in competition). And then they say, "if you go back and look at my original paper, it actually warns against expanding the mandate of this tool beyond our narrow focus of study". Did you really expect people would respect that warning? "Oh no, but what can you do?"
What typically occupies the bottom third of this screen, below where my gaze is the most comfortable? A tilda pop-up console bound to my Windows keyboard menu key.
I have a custom user style that adds white space to the bottom of every web page so that I can maximize FF on this monitor, pop up the Tilda window over top of the bottom third, and still scroll the bottom of the web page high enough to not be covered over.
And then I have my landscape monitor to the left, all within the optimal attitude wedge. In fact, the combination of the two is much better ergonomically than having them both in landscape mode, which was so wide that I used to sit tilted to one side or the other, putting strain on my back (also pushing more of my pixels into the far margins of my vision). I never been happier with any previous monitor setup, though it did require switching from Ubuntu to Mint with extreme prejudice.
In my opinion, most people persist in using fonts that are much too small, I suppose so that they can crowd more stuff onto their desktops. Small fonts would be a problem with this setup as it would cause me to lean forward sharper reading, and also creating sharper viewing angles toward the edges (my input box is presently displaying three lines per inch; I can read what I've composed without difficulty from six feet away).
A portrait-orientation of your monitor makes that objective difficult to achieve.
I suppose if the sum total of your ergonomic wisdom comes from a fortune cookie ("Eyes level with bezel last a lifetime.") and you have no capacity to think for yourself, portrait mode just won't seem terribly appealing. When one's approach to ergonomics is more holistic, one quickly comes to a different view.