I have a relatively slow ADSL connection, it's not unusual for me to be able to download 100% of the file at say 95% the theoretical rate without uploading anything. If the network has enough upload capacity to do this, does it really need my upload? (Note my client is still there if somebody needs a rare block)

I remember some Bittorrent networks circa 2005 or so which tried to monitor you and punish you for not contributing and this was a disaster for me since my upload is necessarily a small fraction of my download. What I found is that kind of network seemed to perform poorly even when I had a good bidirectional connection. As I saw it, people who have the ability to upload more than they download are a resource that that kind of network can't exploit if everybody is forced to upload as much as they download.

You are not required to seed immediately, nor even the same asset. Unless the rules have changed, you would usually just be required to maintain a particular minimum seed lifetime ratio that is easily achieved by idle seeding a library.

The point is to ensure network health with a metric that is simple to understand and verify: that you have been a productive. If you aren't seeding, someone else has to pick up the slack and the network didn't benefit from you obtaining the blocks.

The community itself benefits by giving members a guarantee that stuff there is available with abundant bandwidth, instead of relying purely on the unpaid goodwill of others.

You can't upload much of what's rarely downloaded. The usual way to help the network is to download something popular that you don't need and to seed it. This removes some strain from big seeders and helps them seed other things you don't have.

But personally I believe you're not obliged to anything in torrents, especially with adsl. We got you covered with our multi-hundred u/d ratios. Everyone participating honestly and comfortably is enough, even if that means no upload. Just don't turn it off out of unnecessary principle after downloading.