A swarming prerequisites for modem user is the existance of supernotes (or named Ultrapeers for Limewire). A Supernote will protect a modem user from backbone traffic and so enable the possibility of using this small slice of bandwith for swarming (swarming upload).
For sure a modem user can download multisegmented... as long as he has enough bandwith! It makes no sense to use another download stream, if the first stream does allready use the whole available download bandwith. Same for cable/DSL users, e.g. XoloX makes sure it does not use more download streams as necessary to use the available download bandwith.

Greets, Moak