I wonder how many of those are just mirrors of Linux distros and other open-source software, how many have more interesting things (including software), and how many of those were deliberately configured to be open for sharing. There is the somewhat-well-known filesearch.ru if you want to look for things on this non-HTTP part of the Internet. (If I remember correctly, Google used to index FTPs too and you'd get plenty of results with the right queries, but that seems to have mostly and silently disappeared...)
Actually most of them. It's pretty easy to eliminate them if you have the hostname entry from the scan and cross-reference with public distro mirror lists. Also exclude any edu servers and names of universities etc. If you want to look for actual files, there's hardly anything better than http://filemare.com. Crawlers like Napalm (http://www.searchftps.net/) focus more on servers that are meant to be public. Using filemare one can find the interesting things ;-)
There's still a uprising number of niche ftp site around: MSX, Demoscene, etc. Mostly older scenes that pre-date the WWW and are still around. I think it might be useful to test every port on every ip to see what happens protocol-wise. Limiting to just common ports is probably missing lots of cool things.
According to Reserved IP addresses there are 588,514,304 reserved addresses and since there are 4,294,967,296 (2^32) IPv4 addressess in total, there are 3,706,452,992 public addresses. There are 65536 ports, so one would have to scan all those ports on all those addresses.
