I think a major difference here, between instant messengers that have such support and irc client (fex mirc) that do not, is both the protocol design and the intended use. To be very brief...
IM clients are designed for direct one on one conversations/voicechats/webcams and so on. Yes they have grown to allow things like conferences and group chats, but such things are created by inviting only and are not publically open. IM clients are also designed to be graphically visual rather than just text based, this is part of the protocol and clients initial design and intended use.
IRC however is designed to be a text based CHAT ROOM environment, while both clients and servers have grown to support restrictions (such as with services or channel modes like +i), the protocol design is for an open communications environment and is by default completely unrestricted.
I could go into much more detail about the designs, intentions and capabilities, but there is probably no point. Because of these simple differences i think webcams and such belong in IM clients, and not chat room environments such as irc, web based chat, or forums. Yes, there are a few irc-style protocols and clients that have such features, but they are not very popular in comparison... and i can't think of any web based chat rooms (custom or w/ irc backbone) that have webcam and voicechat support.
Dispite all of the above, i also think that webcam/voicechat support would lead almost directly into some sort of built in media player. Which is an area most im client wont even venture, and something i believe would ruin mirc.
Anyhow, just my opinion.
