To me, it's not so much whether a user has broadband or not, but rather the
idea that the user should give consent for their computer to be used in a
manner like this. I'm all for the idea of apt-p2p and I may even try it out
on my machines, but it seems a bit underhanded to me to require users to
opt-out of being used as a peer.
Consider that a new user installing a fresh instance of Trisquel won't even
know this is operating right under her nose, yet is it using her network
resources, CPU and RAM. It would leave fewer resources for the processes she
knowingly wishes to use, and if she is billed per-megabyte of internet usage,
causes her to pay extra for something she doesn't willingly enable or even
know anything about.
Sure, make it an option, encourage users to opt into it, just like users are
encouraged to seed the ISO torrent. I believe it's counter to one of the
principles of free software, namely that we as computer owners are sovereign
over our hardware, to turn this on by default, even if for a noble cause.