They can be used for military purposes, for military police and for upgrading. Seems like enough and upgrading does remove the ability to build them.
I would guess that Theov is simply planning to duplicate (the .../Unit/Art/ folder and .ini file for) whichever existing unit he decides on, rename that duplicate-unit (folder + .ini) as e.g. 'Resource Colonist', and then use the Editor to add the 'Resource Colonist' unit to his mod, replacing whatever abilities the original unit had, with the Colony-building ability. So the stats/ abilities of the original unit -- in this case the basic Warrior -- would be irrelevant.
On topic:
I voted for the 2nd option in the poll, even though it was restricted to Iron -- because Luxes are nice, too!
I generally don't build Colonies if I can get the Resource by Settling near/ beyond it, but in the case of that screenshot above (BTW, is that from Bamspeedy's 'Deity Settlers' story...?), I would definitely (also) have built a Colony. For Resource-tiles which lie just beyond my borders, and when I can build a replacement Worker in less time than it would take to build a road to/ on the tile in question, and/or get that tile within my borders through Cultural expansion, a Colony may well be the cheapest/easiest way to grab that resource.
Case in point: a Gem- or Iron-Mountain will need 1+9T to road (if not IND) just on its own -- but I can
always build a (replacement) Worker in ≤10T, and possibly in as little as 2T if I have Worker-pump. Later on, after the town has built some (more) Culture and the Colonised tile comes within my borders, I then don't need to waste time/Worker-turns on roading it, either.
Similarly, if I've rolled a tropical/ Jungle start on a Warm/Wet map, building Colonies on nearby Jungle-Dyes/-Gems can also be a good use of Workers (or Slaves) in the early game, in my view. I get the Lux as soon as the Worker has roaded to and Colonised the tile (which also takes
much less time than clearing it by hand), and I can later follow up with a Settler, who can use the Colony(s) as roads (as described by templar_x), and plant a town just beyond them: that town can then immediately use those now-cleared (and roaded) ex-Colony tiles.
I don't build Colonies a long way from my borders though -- when I first started playing Civ3, I played too many games where the AI plonked a city down next-door to my remote Iron-Colony. And I don't bother in the late game either, because I usually don't need them (or can't build them) by then. My Mod-in-progress, like Kirejara's, will have/has more unSettle-able terrain though (Tundra, Deserts, Jungle, probably Forests), so Colonies will be (much) more important...
Outposts? Used them in my first ever game of C3C (English, Regent, Standard Continents), never since. Radar Towers -- never in C3C, but planted a few in my current PtW-game (Germans, Emp, Standard Continents). Airfields -- only if I'm going for invasion, i.e. Dom/Conquest (lots on my turf, 1 on theirs = multiple airlifted reinforcements per turn).