Customers are using...

Honestly we don't know what they are using, really!

SSH/FTP/IRC/HTTP/SMTP/NNTP are very common
People are using those without even noticing

They could be trying more exotic stuffs
Site-locals
Router renumbering
IPsec
Site-local anycast
Translators, other transition tools
Note all of the current customers are dual-stack sites, otherwise they won't be able to query DNS