One spin per desktop works with slow internet and allows to not waste gigabytes either.
I don't want to download a 4GB iso with all major desktops in it - multiply by distro versions and 32bit vs 64bit, and I now need to buy a hard drive to fill it with junk.
I don't want to wait half a hour for 4GB of mostly crap to write to the USB 2 stick.
I don't want to bring a very small iso, but download 1GB at 100KB/s with wifi brown outs.
I don't want to install KDE or Gnome 3 or Unity and not use them : daemons, registries etc. Some desktops when mixed give you two entries for "Preferences / Screensaver" for instance.