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.
