/me noticed how none of the XMIT guys responded to Spola's post, why? Because he just proved XMIT useless.
And the other issue you brought up, IPv6, well mIRC doesn't support IPv6, so having support for receiving a request from an IPv6 client (which it will have to deny since there is no way mIRC can establish the connection) is utterly stupid. So great mIRC can now receive a DCC SEND request from another user... but it can't actually get the file. Yea I definately think XMIT is great! I mean it is DCC SEND with additional problems, and who doesn't want more problems???
As for other clients not supporting DCC SEND, just because they added XMIT didn't mean they had to remove DCC SEND, if they did that was their choice and not mIRC's responsibility to offer a solution to.