After letting things settle and talking with Lindsay, I've moderated my birth-control position a little bit:
The issue with having lots of babies isn't just the quality of life of the parents, it's also the quality of life of the children they have. And that inevitably goes down with the number of children. Sometimes this is a good thing (it my be good for kids to not have all the latest and greatest toys), but it can also be a bad thing (you really can give your children more opportunities if you aren't just scraping by). I'm not sure college education would be a huge problem (with 8 dependents, you would be expected to contribute practically nothing, and it's good for character to have to work some while in school), but it could be under some circumstances, and the principle certainly applies regardless of whether or not college in particular is a difficulty.
Also, the world we live in is different from the 3rd-world... a lot of our extra income is eaten up by the fact that it's just way more expensive to live in American society than in a 3rd-world country. I still think materialism is a huge problem and I suspect that the number of children a couple can support is usually more than they think they can support. I guess the problem with birth control is that it is really easy to abuse for selfish reasons, not that it is inherently selfish.
All that to say, I'm a little more moderate about my baby plans now. Usually what happens when I hear a new idea is I immediately think it's great and can't imagine why anyone would disagree. Then, as it has time to settle, the initial excitement wears off and I realize it wasn't quite as great at I thought it was. So I'll probably throttle myself back down to 5 (to Emily's relief).