Dictatorial? Benevolent dictators maybe, but Mozilla sure as hell isn't a democracy.

Mozilla did have one version of the Personas Plus extension - v 1.4, back in Nov 2009 - where you could have both a full theme and a persona enabled together.
https://addons.mozilla.org/en-US/firefox/addon/personas-plus/versions/
Heard a variety of theories why Mozilla didn't take it further, but that's the way Mozilla does things. They develop a feature until they're satisfied with it, and then release it. Regardless of what the user community thinks about whether it is complete or not. Every major version up to and including had at least one feature that I would classify as half-assed, and I have yet to see Mozilla go back and full-ass a feature in a subsequent release.

I played with Personas Plus 1.4 a bit back then and found most full themes didn't work too good with a persona running at the same time. After that version Mozilla didn't do anything more toward being able to have both enabled at the same time. At least they did try to have both a full and a lightweight theme enabled at the same time.

If you think that is a big issue, why not download that extension and dissect it, and see if you can figure how to modify that extension to make it work with the Firefox 4+ versions.

See also:

• https://wiki.mozilla.org/Personas/Personas_and_Themes (lightweightThemes.forceSkinning -> true)

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold(user set).
• Preferences can be reset to the default or changed via the right-click context menu.