I'm curious what are the limiting factors that make it
undesirable to handle uploads by way of sending
out chunks, round robin, to everybody requesting
something from you.

I personally believe that it could be more desirable,
because although you would be getting a slower
transfer from each source, you would be downloading
from more sources. You would get a percentage of
the available bandwidth now, as opposed to waiting for a
larger percentage.

If gnutella keeps track of upload to download ratio, that
result could be the number of chunks the requesting
user receives at their turn.