While I agree that a good semariton may find it easier to earn the user's trust by verbally asking them the version of their client, this is simply not practical in most situations. In fact... most users who download and install scripts that do re-write the version reply, have no idea they're even using mIRC, let alone what version. "I use Scoop 2.5." "I use Invision 2000." "mIRC Gold."
Additionally, for IRC servers to successfully prohibit thousands of users from accessing the network dangerously, requires a certain level of automation and placement of trust. For the same reason that your browser tells the webserver what version/os it is, your mIRC client makes this information available when it is needed. The only difference is mIRC only gives it out when asked.
I think it would be a very good thing if webservers denied the access of vulnerable Explorer browsers, actually. But that's another issue.