Working with Mibbit clearly costs freenode time and resources that they would rather not spend. Since Mibbit makes money, perhaps you should consider entering into a contractual relationship to provide them with financial incentive and other guarantees, in exchange for their investment in supporting your service?
The 'abuse' reason I believe is a red herring. Abuse is easy enough to deal with, and them operating their own web client won't change how often people abuse using a web client.
The abuse from Mibbit has been the only reason (in months) that I've had to set myself as an operator and ban users. As a user, I don't feel this is a 'red herring'.
So, IMHO, a sad day for Freenode and all that they used to represent, but an opportunity to create a new haven for hackers.
I'm perfectly happy on Freenode, and our IRC channels are quite vibrant. I hope you can reach a resolution with the Freenode staff.