I find colonies useful.
I game style I sometimes play is to only have a limited number large productive cities and go for a Space race victory.
If there are resources that I lack outside my borders I will establish a colony on them guarded by a strong defensive unit.
Mobile offensive units patrol destroying all towns, settlers and other enemy units in the vicinity.
Other civs will constantly send units in to attack and found cities.
I treat this area as a super barbarian farm to train units to elite status and create leaders.

I use these leaders immediately as you can only ever have one leader at once.
This stratigy is only possible if you have military superitory.
It is extremely productive at producing leaders and has a lot less micro management that a total domination game.
The times I have played this I was a democracy but never had any war-wariness perhaps because battles were fought in no-mans land and I had numerous luxuries some attached by colonies.
The civ farm strategy works best in an area with resources because AI civs like to settle there. A rail line to a city with a barracks is also good.