Some ways I think it detracts, going back to an earlier post of impaler's:
It will be atleast as difficult to asimilate a well developed enemy world as it would be to develop your own colony from scratch.
Countries waged war against existing countries often because they wanted to either take the building blocks (stones, jewellry, etc) or inhabit the cities themselves - because they were already built. I know we're dealing with different races here, but the realism argument can be presented both ways. The ultimate answer is that we can do it the way that makes it more entertaining, then write excuses around it. War should produce spoils for the victor - it almost always has. We should not penalise the colonising of a new place just because it's been inhabited before.
Another though, some people were very strongly against farming and morale when we started this project. It wasn't fun to deal with starvation, and it wasn't fun to deal with rebellion. People don't enjoy things that have a negative affect on their colony. They like to feel that they are moving forward, even if their opponents are moving forward quicker. Biospheres will add another layer of limiting that people will be frustrated about. We already have one population cap, we don't need to add a second. If we don't want colonies to grow too fast, then lets slow the population. Adding bio feels like slowing the population down in two ways (population growth & bio maximum).
Basically, I still can't see what fun this adds. Or if it's to limit some problems (such as those Aquitaine said weren't important yet) why this is the best solution compared with alternatives we haven't considered?[/quote]