Not much plugin per se...more like "client for those who can't/doesn't want/other install normal client" Also it's sometimes used as quick way to provide IRC chatting on web pages until some better way is found, so it's not that crap per se, there are just better ways than that
Even if you don't want to install an IRC client on a given PC, you can always use wsIRC which is basically a full-feature online IRC client.
Mibbit is an attempt at connecting all IRC networks into a uniform chat mainframe and... well, it works as it works. Besides, IRC is considered "something for the oldies", access to it is the first hurdle you need to jump over to prove yourself worthy of the honour.