The main thing I found that the extreme environments do is to create the possibility of a phased colony rush (or multiple rushes, depending on how you look at it). It basically makes you have to decide when to make the major investments necessary to start expanding into new habitat types, at least if you're a long-game fan like me. It might be moot in shorter games kind of like CraigHB says.
I don't vary my map settings very much, and I've still had games where the later rush phases played out very differently depending on proximity to the neighbors, early wars, rushing for resource nodes, etc. 'Small' differences can matter a lot, like whether you have the Korath near you on the map and when they get around to using spore ships to attack you. On Immense maps, I'm often still planting extreme colonies well past turn 100 and a few times I've seen a Korath empire actually strong enough to startle me by taking a big handful of my tender new worlds in very short order. (Mostly my fault for not keeping a closer eye on them, but that's a different discussion.)
