As someone who is an ircop on another network with a few thousand users that also blocks Mibbit, abuse is not a red herring at all.
The last time Mibbit was unbanned on the network, it was used repeatedly for users to evade channel bans and harass users. Implementing a non-standard protocol that only Mibbit seems to use is not a valid answer to this problem.
IMHO, its a sad day when a single, relatively new client (that displays ads/produces income for), feels that it is important enough for a network that has been around much longer to bend its policy to work around. And then have the gall to accuse the network of wanting to make a profit itself?