If my neighbor is in heavy jungle, I might just let him clear a bunch of it before I attack - that usually means waiting into the ADs. If it's a religious nut, I might wait until they pop a profit and build a shrine.
Usually, I just go smash in teeth if I'm able to.
I'm confused by the question "Is the war just about acquiring space?" I don't care about space - I care about how many tiles my empire can work - hammers, commerce, and food for GPPs. If my economy can handle adding 10 more cities (and by "can handle", I mean "units won't go on strike"), I add 'em by whatever means necessary. Even if I lose 10 turns of good research, 10 turns of mediocre research, and 10 turns of bad research, I just added 10 more cities worth of research to my empire, plus a significant amount of production. I'm probably just arguing semantics with this, so feel free to ignore me - but "space" has no value, so I don't need or want it.
Back to the original question, some other benefits of conquest vs. magical elimination of your island buddy:
Not only do you save the hammer cost of a settler when you take thier city, you also start with more than 1 population. This can be immediately used to work tiles or convert into needed production (a courthouse, for example - extremely important when expanding!)
You save the hammer cost of whatever buildings survive the takeover.
Not only do you gain workers, you save massive numbers of worker-turns - all those roads and improvements you don't have so spend your own worker's time on is a big deal.
It allows for a much faster expansion of your empire - you couldn't afford to REX those 4 or 5 cities because of the economic impact while you wait for courthouses - but you the pile of cash you get from sacking the city plus the whippable population into economy-helping buildings lets you immediately absorb them and turn them into a positive-gain on the economy within a matter of 5 or 6 turns after the revolt ends. When else in the game can you get a new city and have a courthouse and a granary in it in 5 turns? Only if you're running Universal Sufferage and have lots of gold is this even feasable.
Early conquest, middle conquest, or late conquest - if you can pull it off without units going on strike, 95%+ of the time it is worth doing. Modern conquest? Well, only if you're going for Conquest or Domination victory. Or they have aluminum and you don't and you're going for Space Race. Or they have the Space Elevator near your territory.
Yeah, yeah, I know. I look for reasons to conquer.
