Having your IP/Host publicaly available and visible is a big IRC weakness, and it's noted in the problem sections of the IRC protocol (RFC 2810-2813).
Remember the IRC is 15 years old. It was though of way before the spam days. The idea of IRC was to allow anonymity, and the way they thought of it was to use a nick as a label and your host to reach you.
You IP being visible, a lot of jobless people will just use it and try to DoS you, or do similar attacks on you (don't ask me where is the fun in that).
Most similar system will use other methods of identifiying you in which your host is not visible, and only send your IP to those who need it (for direct connection, the equivalent of the DCC for IRC).
Undernet solved this problem in some way. Once registered and authenticated to X, you can activate the +x mode on yourself, and from that moment your host is replaced by a string like username.users.undernet.org, effectively hidding your host. I've seen networks on which the host is never displayed, they are replaced by some string composed of your nick or your real name or the like. I guess other networks are also proposing methods of hidding your host.
(As this may be going to much technical, please contact me by e-mail if you which to continu this discussion instead.)