Apple charges so much for RAM or storage upgrades. I used to buy the lowest spec, toss the OEM parts shortly after purchase and upgrade and come out way ahead.
With the computer you're looking at though, the RAM isn't upgradeable. I don't think there are many harddrives out that fit the space, either, so they kind of have you locked in to upgrading at time of purchase. It isn't all malicious. It's more sealed as a side effect of being so thin. Also, I think system requirements, especially with the proliferation of web apps and smart phones, aren't as important to future proofing as they once were.
That said, if you plan on keeping this for a year or two, get 8. If you want to keep it longer, I'd spring for the 16