This was considered, however after much testing I decided to make it optional.
As you know, Windows 7 has IPv6 support enabled by default, even if you do not have an IPv6 internet connection. It is not possible for an application to detect whether you have an IPv6 internet connection, at least, not without attempting to connect to a known IPv6 server, which is not practical and in any case might not be reliable.
If mIRC enables IPv6 support by default on Windows 7 and it attempts to connect to, for example, irc.efnet.org, the getaddrinfo() request will return a pool of both IPv4 and IPv6 IP addresses. Even if mIRC prioritizes IPv4 addresses, it will still have to cycle through the whole list of IP addresses when attempting to connect to a server, a third of which will be unreachable IPv6 addresses.
We already receive so many requests for help from users who have trouble connecting to IRC servers. Considering that the number of users with IPv6 connections is incredibly small, I think it would be it be unfair to force 99% of mIRC users to have to deal with connection issues/delays due to unreachable IPv6 addresses.
That is why mIRC does not enable IPv6 by default and why it warns users, the vast majority of whom are not technical and do not have IPv6 connections, that they should not enable IPv6 support. I too would have preferred to enable IPv6 by default but it just does not seem practical at this time.
