I do agree that ideally it should be enabled by default for Windows 7, however when I introduce a new feature that has the potential to affect large numbers of users negatively, I usually make it optional and wait to see how it performs for those that use it. This allows me to resolve any teething issues. Once it looks like it is stable, I enable it by default in a subsequent release.
My main focus for this release is mIRC as a Unicode application. The conversion to Unicode has required huge numbers of changes, so I it's important to ensure that it is stable before introducing other variables. But yes, IPv6 will be enabled by default in a subsequent release.
