I basically have three reasons why I think logging in on irc is more convenient than on a web form.

Firstly, on a web form, you have to enter your irc nick separately, which is error prone, whereas on irc you automatically cannot log in anyone else than yourself.

Secondly, the users only have to set up one application, not two. Many irc clients have at least some scripting support, so a user could even fully automate that when he goes on irc, the client joins the cbstream channel and logs in for him. Similar is true with a very dumb irc client: I have used irc to get support from an install cd that only gave me a busybox rescue shell. I could use the wget nc command to submit a simple web form, but if you already have a connection for irc open, it's easier to just type one or two commands there than mess with a web form.

The third point is the convenience of implementing on the server side. It would be possible to write a cgi that runs in the webserver, and make it transmit the data to the cbstream server, true, but it seems much easier to make the cbstream server accept one more kind of input as it already has the irc connection open. This is especially true to the new channel-based login mechanism, which is actually even easier to implement than the memoserv-based one, only I didn't realize this back when I wrote the memoserv handler.