mIRC providing proxy services would only give the "hackers" an easy way to hide their own identities. Most IRC networks have usermodes that allow the user to hide his IP address in a way that prevents ban evasion on the network and of course allows the opers of that network to see the real IP in case any further action needs to be taken against a user (ISP complaint, Z-line, etc.).
In other words, any IP masking should be done on the server-side, not provided in some way by any client software. And for many many networks it already is.
As far as the overall idea of being hacked goes, the reality is that whenever you are connected to the net your computer is vulnerable. In fact I'd say even if your IP is visible on an IRC server and someone does pick you to try and exploit you're probably still more vulnerable to being breached by some worm that randomly attacks you than the self-proclaimed "hackers" you'll find on IRC.
IRC should be a safe place, however it isn't mIRC's place to provide this kind of service. If your network doesn't offer IP/host masking ask them why not.
