And I vote for this too. I live in the metric part of the world and I'd like to see both vales, without keeping the conversion chart in mind. Thanks for the vote and other issues you supported.

Professionals, like myself, use Illustrator. Hence my request for the feature.

These options are considered to be advanced for new users, not professionals, and therefore hidden.
I too initially asked about separate fields, so I could control both sizes: em-box all the time and x/cap depending on my choice. But the team decided nobody would need to see both sizes at the same time.
Considering the height of Illustrator Bugs mountain and depth of Obvious Lacking Features ocean, I’d leave this as it is now, honestly., except for the Ai remembering what option I picked for each text object after the file is reopened.
Voted anyway.

Not unusual behavior at all. MANY industries use inches(or mm) to set text height while still needing point sizes. Why would this be considered too "advanced" for professionals?

Well, Derek, font height units are the same as type units, you can’t set them to anything else. I personally just use mm as type units for many years, before I proposed this feature, so I didn’t think it through well enough, I am sorry for that.
And while you request has a reason behind it, I think this will not happen. See, the team has decided to hide these options, rightly assuming them to be advanced. So we can’t have them appear in Preferences when it is disabled.... (and it should not be placed anywhere else)
Sure, we can have this drop-down magically appear when the font height options is enabled! :D But this is extremely unusual behavior.
At least Ai allow to enter you units right there in fields.

Nope. My preferences are already set to inches. That doesn't affect new Font height option units. They are always points.
*Edit, I don't want to change my regular font units, just the new font height options.

You can do this already in your unit preferences. Preferences>Units>Type