Voice/video support being added to mIRC itself would mean a much larger portion of IRC now have support for those, rather than a script. Even the extremely popular scripts like nonamescript have a tiny userbase in comparison to mIRC itself. Scripts would not be very widespread at all which would make finding people with a client that supports voice/video a chore.
Also, if mIRC added it I could see other clients following suit. I've seen discussion forums for other clients and mIRC is very often used as something to aim for. "Add such and such like mIRC", "mIRC has blabla, please add it", etc. And with it being the most popular client, people will want to offer what mIRC has plus more.
As for the script option, I have seen scripts offering emoticon and webcam support but never voice. Got a link for one of those?
Regardless, I doubt very much it's an implementation that would even be in the same league as one built into mIRC itself.
As for the useful features you'd like to see added, what are they? Voice, video and emoticons are all very popular suggestions on this board. In fact I can't think of many that are more popular. When thinking of useful suggestions, you should try to think of what many people would use rather than just yourself. For example, I have no use for UTF8 support but recognise it's a good feature because it makes mIRC a lot more accessible for those that use different languages.
