Subordinates we control take actions that benefit us, acquiring provinces, building up True cities, expanding forests into the steppes.
Kinda-sorta. Subordinates we control take actions that satisfy their purpose, which may or may not benefit us. Colonies in particular take a bunch of land development actions (new settlements, walls, roads, forests, cities, etc.), which are absolutely wonderful if we are planning to integrate the land, but kinda useless if we aren't.
(For reference, I made a
compilation of subordinate actions about a week back; feel free to take a look.)
Moreover, we're on the cusp of both a Golden Age and a new government level, so we may be able to integrate them fairly soon, assuming we take some admin technology innovations, and that integration will pay back a significant portion of the invested stats and provide us with additional province, king, and guild actions. I'd say it's fairly worthwhile.
If we can integrate them and gain the benefit of those invested actions, then yes, you are absolutely correct. Especially since stats invested with Support Subordinate and Influence Subordinate mostly go to the subordinate and not just disappear, so if they can put it to good use improving the land further that is yet more reason to keep them around.
The problem is, I very much suspect we won't be at the point of integrating our Western colonies anytime soon.
For one thing, we don't currently have the room. It is
possible that our gov't improvement will free up space for more colonies, but it is possible that it won't, or won't free up much. Even if it does free up room, it would have to free up a LOT of room, especially since we are thinking of integrating Txolla.
For another thing, we literally can't. We've been told that without 2 of better roads/boats/horses, we won't be able to do the integration. We can maybe get better roads, but not for a while; no idea how we can get better boats or horses any time soon.