If it's so vulnerable I'd just station an army there rather than hope the small force limit they'd be able to raise.
The point is not having to turn my attention to the area at all and it should be noted that whole areas are worth minor handfuls of warscore and aren't worth defending when fighting in Europe. The last bit being why most of the European colonial empires had provincial troops which is best represented by Colonial Nations having their own armies.
Also, their force limit wouldn't be all that bad as they wouldn't be getting the Autonomy cap and could field the full force of the area which I have seen the native get to 50k plus and the high end being well over 100k.
Reasonably sized navies around the Cape and Magellan Straits can effectively block any major invasion from Europe into Asia by Western/Eastern route. No need to create extra CN.
As I said above, the point is to not need to direct my attention to the area. Further the issue is much more often the nation I'm fighting already having a troop or three in the area as the AI isn't any more willing to actually send troops to Africa, Indonesia, etc then I am and is instead just being bad about where it keeps it's armies.
That last point being a good one for why the Colonial Nations would also help the AI as well. It isn't all that rare to see the AI getting large numbers of troops stuck in the arse end of the world with no way to get them back quickly, while leaving their European territories in danger.
Even English, with their enormous success in India, relied mostly on local elites to rule, and on local soldiers for control.
Not unlike the system expressed by Colonial Nations? Ruled by their own elite with some minor oversight, check. Local armies, check.
While the Colonial Nation system isn't a perfect recreation of that, as it is purely the creation of a new entity as is, it could probably be changed slightly to allow existing nations to become a Colonial Nation, or Colonial Nation like thing if they wished to more realistically model the system.