If the main decision that the user had to make was a) how much disk space am I willing to contribute to bitcoin [knowing it can be reclaimed anytime if needed], and b) how much bandwidth do I want to contribute, then we could probably get users to make the right decision every time without burdening them with the details.
That generally results in "what am I getting out of this? Uh. I don't care. It works with zero right? Zero." I'd rather express bandwidth settings in terms of rate limiters. Setting yourself down would reduce your contribution, but would also reduce your consumption. Setting it high makes you contribute more but also makes your Bitcoin update faster. So even someone with a very simple understanding of the option knows that he's getting something out of cranking it up. we have this with listening today, without listening you "only" get three bars. To get your >8 connections and four bars you must listen. And people do go out of their way to setup the port forwards just to make the gauges go up not because they're trying to help the network, but because they expect it will make it work better.
I fully agree with you, it sounds like you're suggesting I've proposed something poorly thought through, when I only provided the simpler explanation in response to your objection that it otherwise was too long for the average user. As noted previously, I also propose the default being "altruistic", and of course, this setting would be something buried on a settings screen, and not something thrown at the user when they install. Most casual users won't even see the setting let alone think to slide it to the minimum.
That said, users should have every right to go to a settings screen and choose the minimal contribution for any reason or no reason at all, no questions asked. Their computer is their property, they have the right to restrict its resource usage as they wish, and software whose developers insist on consuming more resources than welcomed is as annoying as telemarketers who think that nobody will miss the few wasted seconds of their day when they call people. They are the reason non-developers value "walled gardens" like Apple's App Store, to the befuddlement of many developers.
Allowing a user to get more "bars" in exchange for contributing to the network is very intuitive and demands none of the user's unoffered attention and is consistent with what I have in mind.