If you play enough the AI will.  Depending on who you are playing and have the AI play as.

That is the point, I know it uses my unit designs, but it will only use them as magnar, if I have played magnar...
And some factions are just too... Not my style for me to enjoy playing just so the AI have better units.

Sincerely
~ Kongdej

All of these are fantastic suggestions
I'm just too lazy to make all those units.

Sincerely
~ Kongdej

You would have to make some intelligence when coding this part I know, it will take some time and some advanced thinking, it was also just a cry out to actually put in more unit designs in the core game in which I already said the game needed before release

I hope for some sort of unit design pack, while I agree a community pack might be difficult to handle for new players, just make it so they have to go and download it off the forum to start with. The reason I put the idea for the AI trying to leech off designs would be, sometimes it might even field a few poorly designed units, but it would feel more alive while trying to convert your great idea into its own units.
It would take some time to code up, that is true, if the devs can find a better use of they're time I at least hope they see this note, and patch the game with 6 more unit designs for each faction, preferably some unit designs that aren't rubbish. (I very very rarely use the predesigned units, have used a "Militia" in 1 out of 20 games, and a predesigned scout in 1 out of 10 games, and thats about it, I always find most designs lack traits, or focus, and some tactics are completely avoided in the predesigned unit section.

Sincerely
~ Kongdej

A unit designer linked to the factions would be a nice addition to the current workshop. I wonder why it isn`t there already. Would be nice to make custom factions totally unique, but i see a problem here. You`ll have to count in all the possible situations that random maps and faction AI will bring with them -  resources and tech priorities.

Personally i will love to spare few hours for creating various designs for my custom faction even if this will produce huge amounts of the unit types

EDITED:

Just realised that this can already be done via the cheats. Start a new game with a faction you want to provide the extra unit types and use a cheat that unlocks the whole techtree (if there is such a cheat). Design as many units as you want. The next time you`ll play the game against that faction - it should have those designs available to them.