Subtleties of behaviour also have effects. An example of one that bit me, is that the Undernet IRC server considers 1 IPv4 address equivalent to 1 IPv6 /64 for rate limiting purposes. While this kind of heuristic works OK with the same values for private and public servers both on NATed LANs and the intartubes for IPv4, for IPv6 everyone on your LAN connecting to your private IRC server will have an IPv6 in the same /64 and rate limiting will kick in unless you completely change the rate limiting values. So that is an example of an application which *needs* to know the type of address in order to be able to make the correct decision.
Speaking as the person who actually designed that system for Undernet, that behavior was intentional. :) ircu knows about 6to4 addresses and teredo addresses, and will rate limit them based on the IPv4 address contained within them to prevent people using 6to4 to avoid rate limits or glines/bans. Also, since changing your "autoconf"'d address is near enough trivail, people using them for ban evasion, and flooding is a major concern, so there is the intentional per /64 rate limit. ircu is primarily designed for use on Undernet, which is quite literally constantly being attacked, both intentionally, and by misbehaving/misconfigured clients. Without strong safeguards in place, the servers melt. (actually, even /with/ strong safeguards in place, the servers often melt anyway) ircu internally knows the difference between v4 and v6. The major reason Undernet hasn't been upgraded to v6 is that our services haven't been updated to deal with v6 addresses being announced by the network[1]. While I understand this is annoying for private installations of ircu, there are tunable options that you can tinker with. [1]: If anyone's bored, I'm happy to give you the code and help you through writing the code to update it for v6, I'll update Undernet to run the new code, and we can finally turn on v6 network wide. I warn you this is perhaps not as simple as it sounds at first glance to get it to the point we can turn it on network wide.